@Pam
My first visit to England was mid-July through mid-September 1977. Per my British friend's advice, I packed for a "California summer" since that is what she experienced in East Anglia in 1976. As I recall, it rained sometime nearly every day I was there in 1977. My mother was visiting for a month in the middle of my stay and I asked her to bring me another pair of trousers and a long-sleeved top, and she took home all my shorts, and a couple of short-sleeved t-shirts I had no use for. We all spent a day on the east coast in borrowed mittens and hats in mid-August.
On the other hand, my husband and I walked the last bit of the Cotswold Way out of Bath in October 2011. We were frying in the heat. Had a friend visiting India at the same time and he said it was actually hotter that day in England that it was in India.
Still we're gambling on a moderate summer this year and haven't booked any accommodations with air conditioning. Kids, however, do not think "this too shall pass", so if you haven't booked yet, it might be good to get a place with A/C.