I would avoid UK half term in May if possible as it could be busier then perhaps.
Does start to get very warm in June but if you stick to getting to the parks early and going back in the evening routine you should miss the worst of the crowds & heat.
To be honest it's many years since I went in June but I would be surprised if it's very busy - generally July is the worst for heat and crowds.
Early May would be better, but maybe not convenient for you.
(take it by your TAG you are a blue by the way?) hope you don't mind advice from a red then
We are going as a family of five on the 26th May 07 - 16th June 07, The children in the US will already be on their holidays so it will be busy from sometime in May until they go back Mid August (I think it is Mid August?). Good luck with your planning. May is a good time to go weather wise, June can be very humid,anytime would be a good time to go in my eyes.
Only been once in June & once was enough!! As previous poster states "Gay" weekend happens about the first weekend in June -happens every year but for some reason WDW don't tell anyone! So expect MK to be ultra busy! We found every park busy, on one of our days in MK it was open till midnight & the quue for Space was still 90 mins at 11:45pm ! We have had friends go in May & the weather has been good & queues very light but if you can only go in June then that's when you can go..
SD