If you can afford it, go for ploycotton tents instead of polyester. They're much cooler during the day and retain the heat better at night. Also quieter in the rain. We picked up one of last years Kampa Filey 6 Classics for £650 when it was retailing at £1200. Sewn in groundsheet, sleeps 4 very comfortably. Headroom is about 7ft, canopy on the front about 4ft deep, living area approx 8 ft x 12ft, sleeping area 8ft x 6.5ft split into 2 seperate pods. Brill tent!
By the way, a hydrostatic head of 1500mm is classed as waterproof. So anything above that will cope with the british summer, but over time, the "waterproofness" decreases, so start with the highest hydrostatic head you can find as it will last longer.
