If you are within travelling distance of Dorking, there is a fab outdoor party for children called PartyTank. Kids get to have a turn at archery, paintballing, rifle shooting. They get taken for a ride in a jeep & do proper off-roading & then the piece de resistance is a ride in a tank. Prices start at £15 & go up to about £20 per child.
It was all very well organised & felt safe & the children were never left unsupervised.
I took DS & 3 friends yesterday for his 7th birthday treat & it was a huge success.
You can find it all at www.partytank.com