Information about the EEA permit and other aspects of being an expat in England can be found within the Guide to London.

More specifically in answering your question, I've heard from other expats that things as informal as Valentines/Anniversary cards, pictures of you together at important events/vacation, etc. Other ideas:

Certificate of a joint bank account
Letter detailing your relationship - how it began and plans for the future.
Affidavit of Cohabitation - written statement by two people that they are living together, signed in front of a notary and used when obtaining insurance (medical, dental and life) for policies allowing domestic partners to cover the other.

I would also recommend checking what other expats have done by perusing questions & answers in the forum:
eea-family-permit-t28034.html
info-family-member-non-eea-of-an-eea-family-permit-for-uk-t12107.html
eea-permit-some-details-clarify-t31039.html

Check to see what they used to apply, and what tips they may have. You can even connect directly with many expats through the network (once you've made a few posts to prevent spam.) I also tweeted your question so hopefully we can help you find the answer to your question. Best of luck!

 

I am going through this process and wrote about the steps we took and documents + proofs we submitted. I hope it helps others who are doing the same: http://www.anaelisamiranda.com/1/post/2 ... lgium.html