I am from over the pond, as you might guess from my forum moniker! The weather is very varied in the British Isles even during our so-called summer. One summer I took my husband back for a visit and it was in the eighties and sunny for the whole three weeks we were there, not a drop of rain. He never believed in "always rains in England" after that. So far they have had lots of rain and floooding, but are hoping for better weather for the upcoming Olympics.
Take some rain gear and a brolly (umbrella) if you have one. Pack some light summer clothing but also take a warm jacket and a light weight sweater or two with long sleeves.
Have a great trip and may the sun shine on you the whole time you are there.