iPhone locked to T-Mobile US work in T-Mobile UK?

Hi all.

I have just gotten an iPhone 5C, which was bought in the US. I am in the UK. The iPhone is locked to T-Mobile US, and I was wondering if it will work with a T-Mobile UK sim or not? And if it doesn't, will I have to contact the American T-Mobile to unlock the iPhone?

I hadn't known if they were using the same sims etc, as they are the same company.

Thanks.