tescos actually do a suprisingly good range (branded sherwood) with their 4 person coming in at under 50 notes , and the 6 person at arround 65
if you want to spend a bit more than that it is well worth checking out your nearest go outdoors which will have a major range of tents on display for you to go in and out of etc
see this link for your nearest store
Great Range of 5-6 person Tents at Excellent Prices at GO Outdoors
Ive just bought a 1 person vango there reduced from 100 to 65 notes - but they have a good range of bigger tents too