Sprint iPhone 5 and using it in the UK

Hello everyone! I'm going to be traveling to England in a few weeks and I am have a few questions. First of all, I have a Sprint iPhone 5 and I was wondering how difficult it would be to unlock it through Sprint? Are they annoying with it?

Also, I wanted to know what the best prepaid cellphone company in the UK to use? Like will I get any LTE with any of the carriers in the UK? And how will I go about getting a SIM card and what is the best plan do they offer?

Hopefully someone can help! Thanks 😀