What should I pack in my suitcase?

In 3 weeks I will be in London, England not just as a tourist either. My sister, Jessi, is getting married to Roger Day on July 17th.
I will be there from July11th-July18th. I'm excited to see Jessi and where her and Roger will be living. I'm also excited because I've never been to London, I've been to N.W England (Liverpool & Manchester area) but never London.

So what should I pack? I already have the general items in mind but what are some things that I might never think of that you guys could help me with?

It's only 3 WEEKS AWAY!!!!!!