That's a wonderful time to visit England. I've visited the area in April-May several years ago and did a similar trip to what you're describing (southern England, Wales and spent a week in London itself), and it was awesome.
Note that this year you shouldn't be in London in July-September, unless you're interested in Olympics. That means that people who want to visit England not for Olympics will probably go there earlier than usual, and you'll have all the crowd you want to avoid right there with you.
But don't let it stop you, England is wonderful any time, especially late spring and early summer.