Hi Daniel,
My DW, Jacky, and I are in London (suburbs) now visiting friends & family, and the weather is truly glorious. Clearly, all the prayers helped.
Forecast is generally good, but, as usual, it is Only A Suggestion!
Enjoy London. It's a great city! If it rains, don't be deterred - it's part of the atmosphere! If it's nice, definitely do a city tour on an OPEN TOPPED double-decker bus. You'll see fascinating details of the buildings and the city that you won't see any other way because the streets are too narrow! Have fun!
See You Real Soon!
Richard