B87 Those sorts of temps are not 'exceptionally cold' in May, even in London. Cooler than average yes but it's not unheard of for temps to struggle to get out of double figures in May- even in the south.
May 28th 2007- max of 7.4C at Heathrow. Now that is exceptional.
Looking back over the last few Mays as well- Heathrow has had a fair few days at 14C or below.