To help Christians think about this most important issue here are some websites offering Christian comments.

The Baptist Times has a list of Baptist contributions to the debate at http://baptist.org.uk/Articles/467100/Baptists_blog_on.aspx
Resources have been produced by the Joint Public Issues Team which represents Baptists, Methodists and the United Reformed Church at http://www.jointpublicissues.org.uk/issues/eu-referendum/
Church of England also has a website http://www.reimaginingeurope.co.uk/

I have written a piece on the topic on my theology/ministry/mission blog at www.pbthomas.com/thoughts