Six years into living here, I'd say there are about two weeks a year in which a sundress may be worn. The interesting thing is that you'll have no idea when it will happen. April, May and June seem to have some nice days, and then it gets really miserable and clears up by September to give you an absolutely gorgeous autumn. A nice summer's day means it reaches 70 for a couple of hours in the afternoon so you get to remove your cardigan and feel the sunshine before the clouds return. The previously mentioned actual warm days may reach 78 or 80 as it did for four or five days last year, whereupon the media will announce a heat wave and advise everyone to drink plenty of water and wear light clothes. Most British people will be on this, as they remove their coats in early April, the beginning of British summer, and put them back on in September as it actually starts to warm up.
It's nearly impossible to prepare yourself for the utterly weird weather of Britain. I knew when I moved here it would be bad, but I didn't realize it would be bad nearly all the time except for a few weeks in spring and the entire fall. But who knows, maybe this year will be one of those mythic ones that actually receives some warm weather. Apparently the summer of 2003 was hot. In July. The feat was repeated in 2006 as well, but that summer I managed to go to Spain so I never saw it!
On the bright side, if you ever are lucky enough to encounter a warm day and you are someplace nice, like coastal Devon or something, you will think you've never seen anyplace so lovely in your life.
