Hi,
I'm a Christian currently at Warwick. I usually go to the Chaplaincy service. Everyone is welcome as it is an ecumenical Anglican/Free Church service. Alternatively, there is Westwood which is quite popular amongst students. The service is more lively than the Chaplaincy service but it equally as sociable (and where I go during the hols as Chaplaincy services are term-time only). There are other churches around but I do not know enough about them to comment but if you want to talk to anyone about church/other Christian stuff when you arrive there are 2 separate Christian societies - Christian Focus (of which I am a member and prayer sec) and Christian Union.
Hope this helps.