Weather forecasts are fine, but don't rely on them. I am doing weather dependent work on our house and in the past few days they have forecast dry weather only to change the forecast as the rain approached. There is no doubt that this summer has been quite exceptionally wet and as a result has felt cooler than normal. But as someone has indicated already, Britain has very changeable weather more or less all year around and the rains make it greener than it otherwise would be. The English will suggest that Scotland is wetter and colder than England but this is a gross exaggeration of a very minor statistical difference, England is definitely not in the tropics (that remains the place where I have felt coldest in my life, in truth mainly because of the attitude to heating, which is likely to be seriously necessary whatever the date). There have been no floods of significance in Scotland this summer, although there has been a lot of rain. If you must go to London, by all means do - but if you want to enjoy your visit you should also consider elsewhere.