You have a great capsule going on here. I agree with some of the suggestions above about taking an extra sweater and a pair of extra shoes with you. I went to London last September and the weather was just fabulous. It did not rain even once but again it is highly unpredictable.

You all are bringing things up that I hadn't thought about. I went ahead and ordered the polka dot tights, and I will probably order a plain nude pair as well to go with the dress. I'll go ahead and take an umbrella and also maybe a packable rain hat. I think we'll mostly be indoors, but commuting on foot, of course.

Thanks for the replies, everyone! This is so helpful. Sorry for the late reply.

Oh, one thing I just thought of: if you happen to have one of those Asian style folding fans, you might want to pop it in your bag. I sometimes carry one in the summer because a lot of places aren't really designed for unexpected hot weather and it can be very nice to fan yourself in those situations!

Ana I haven't read the responses so my apologies if I'm repeating stuff.
I was in the UK last year in August. My capsule was very, very similar to yours and perfectly adequate. The only thing different was that I had a lightweight kind of gauzy cardigan, black and a little dressy looking, that rolled into a tiny space in my handbag. It was a) a little more coverage if I needed it and b) could make a plain outfit a bit dressy.
The other item I packed was a lawn caftan that I used for sleep and leisure wear. It was really handy at the end of a long day tramping or driving to be able to flop into something unconstricted.
Manchester was my base too, coincidentally.