You probably already have a phone, most likely a smartphone and not a flip phone like me that existed before the last Ice Age. People in England are not that different from those in the US: they love their phones and probably could not go without them these days, so having a phone in the UK is important.
There is one problem though: you're going to meet a lot of people (no, that's the problematic part), and you'll get their numbers. However, they most likely will be UK numbers. Because you'll be calling/texting them from a US phone, you'll in fact be making an international call, which can incur roaming charges, which generally tend to be significantly more expensive than domestic calls. Thus, unless you feel like paying for international calls/texts, you're likely going to have to modify your phone or get a completely new phone in the UK. I say "likely" because your carrier might provide a method by which you can make international calls or texts relatively inexpensively (e.g. prefixing your contacts with a certain calling code). If that is the case, congratulations to you! You can ignore the rest of this post. :)
Let's first address "modifying your phone" - what exactly does that entail? No, you don't need to go into the internals of your phone and rewire it (and if you have iPhone, that's practically impossible). What it means is that you can change the identity of your phone to everyone, and this is via a SIM card. By changing your SIM card to say a UK SIM card, to the rest of the world, your phone will be perceived to be a UK phone instead of a US one. This will allow you to make calls to other UK residents as if your phone was a domestic one, allowing you to avoid the roaming charges mentioned earlier.
There is one catch to this, which is that your phone must be unlocked for you to change SIM cards. If not, switching SIM cards will cause the phone to no longer work (i.e. you can't make any calls or send texts with it because it will have no reception). So if your phone is actually not unlocked, and you are thinking about switching SIM cards, do look into how you can unlock your phone. Sometimes, unlocking is free. Other times, it can incur a noticeable cost, which you may not want to pay, or maybe you don't want to use your current phone in the UK. Given that you'll only be here for one year, why bother putting your permanent belongings at risk for damage? That leads us to the other alternative.
In the UK, there are many phone providers and carriers, and in Cambridge, many of these providers/carriers have their own stores near college grounds (e.g. King's College) that make it easy to checkout potential phones you may want to use while you're here. There is also a large variety of phones and data plans offered. If you're looking just for some basic phone functionality (calls and texts), there exist very cheap offers for just that functionality. Since this phone that you buy is somewhat temporary (you probably will not use it once you leave Cambridge), if you incur damage to the phone, the personal loss is not as significant compared to say if your original phone was damaged.
Whether you plan on purchasing a SIM card or an entirely new phone, there are many factors that go into what plan you end up choosing and even which carrier/provider you end up buying from. Here are the ones I thought about when I was looking for a phone/SIM card:
1) What sort of calling/texting functionality do you want while you're in the UK?
The plans offered will come in a variety of forms. Some will offer just calling but no texts. Others will offer a limited number of calling minutes but unlimited texting. Think about your current habits: do you prefer to call or text people? How often do you do it? This question extends beyond just texting and calling though if you plan on getting a new phone. Do you want your phone to just make calls or send texts? Or would you want it to be more of a smartphone type with apps, Internet access, and the like? These questions will help you determine what sort of phone you want to buy.
2) How good is coverage for each carrier?
Although there are many carriers, the amount of coverage they might provide can vary, even though they may not say it. Thus, while a carrier may offer a great deal, it may be because they know behind closed doors that the service is very poor. So do some research into the quality of coverage (both in the UK and internationally if necessary). It can be as simple as searching on Google "UK carriers with the best coverage" as I did. This sort of work should be done before you go into any phone store so that you can focus solely on pricing issues when you are in such a store.
3) What are the specifics associated with the data plan?
Whether you are purchasing a phone or a SIM card, it will come with some sort of data plan, which determines what sort of charges you incur when making calls, sending texts, or accessing the Internet.
Some plans are set contracts of a certain duration (e.g. one year or two years) that will provide you with a set amount of minutes, texts, and data (if applicable) every month or so. Other plans are "monthly" contracts in which you get a set amount of minutes, texts, and data that is valid for 30 days. You can then pay a set amount (called topping up) to get that same set of minutes, texts, and data for another 30 days. For such a plan, lookup whether or not unused minutes/texts rollover. Other plans are what we call "pay as you go," in which you add money (as much or as little as you want) to your phone periodically that you then spend as you make calls, text, or use the Internet (each has a particular cost/rate associated with it). How you spend that money is entirely up to you, and it doesn't have an expiration date or a contract.
4) How much am I willing to spend on a phone?
Remember, you probably will not use this phone again once this year is over, so think about whether or not you really want to for example have all of the conveniences of your old phone if it means paying significantly more to buy it. To avoid such a pitfall, try setting a maximum price level above which you will not purchase a phone. That can definitely help in figuring out which phones you want to buy.
5) How easily available is their customer service?
I started doing my research while I was still in the States, making going to the store to ask questions virtually impossible. Certainly, they should be able to provide customer service in store, but if they have easily available customer service online, that could be extremely helpful because that means you can learn a lot more about their services in greater detail without having to be in the UK. It sometimes can also be an indicator of the type of customer service you will receive when you're actually shopping in the store, which sometimes be a huge incentive or deterrent to purchase a phone or SIM card from the carrier.
Common Carrier/Providers
Below is a list of the most common carriers/providers that I encountered while looking for phones. I will not provide any comments on them because the information on these carriers may change from year to year (I found this to be case when I started reading/getting advice from former CME students regarding phones). Some may even go out of business, as was the case for Phones4U which closed its doors in late 2014. In addition, as evidenced by my fellow CME companions this year, the choice that I found to be optimal was not the same for everyone else.
1) O2: http://www.o2.co.uk/
2) Carphone Warehouse: http://www.carphonewarehouse.com/
3) Vodafone: https://www.vodafone.co.uk/
4) Three: http://www.three.co.uk/
5) EE/Orange: http://ee.co.uk/orange
6) Giffgaff: https://www.giffgaff.com/
I hope with these list of possible carriers/providers along with the criterion I provided will help you make a choice on choosing a carrier/data plan and a phone/SIM card! If you do have any questions or are curious about the decisions I made when purchasing a phone/SIM card, please feel free to send me an email via the "Contact" section!