Maybe things have changed, but in all my years of going to weddings in the U.K. with or without a morning dress requirement I have never seen a full-length dress on anyone but the bride, bridesmaids and officiating clergyperson! 4 weddings and a funeral is an excellent suggestion for dress ideas, but really anything that's dressy (and isn't black or white) with a hat will be acceptable. Take an umbrella - there's often a certain amount of hanging around. One difference between US and UK weddings is that, unlike the US, the ushers will escort you to your seat, but they won't take your arm. The first time that happened to me over here it really spooked me