Nowadays, it becomes very hard to live without being "over-connected". When you will be in Great Britain, I'm sure you will need to things:
It's very important to have a UK phone number when you will be in London. Indeed, if you give a French phone number at an employer, he won't call you back if he do not want to spend more money calling a French telephone.
The first choice you will have will be the service. "Do I want a pay as you go service in order to be independent or a pay monthly service in order to know each month how it will cost ?". It is hard to find a plan (pay monthly service) for only three months, and I didn't want to open a plan and to stop it three months later. It's why I choose a pay as you go Sim card for my mobile.
The second question before choosing an operator for your mobile phone is this one "Will I use it to call France or to call other UK mobile phone?". If like me, you often need to call France (parents, girlfriend...) you had better to choose Mobile World, which provide international calls between 0.05 and 0.15 £ (0.07 £ to France). But if you need your mobile phone only to find a job or to call or text people in London, you will probably choose an other operator like Virgin, 3, Orange, T-Mobile, Vodafone, Tesco Mobile...
If, like every Efrei students, it is difficult for you to live several days without checking your email or surfing on the web, you will be happy to hear that it wasn't so difficult to have internet in London.
If you don't want to pay to access internet, I know three opportunities:
The classic way is a little bit expensive... You have 2 choices: