I think that, as far as tents go, what you decide to buy should very much depend on what you want to use the tent for. If it's the odd night out, not too far from the car or even in the back garden then I can't see any problem with spending in the region of £50 or less and getting something that is fairly heavy (2Kg or over) and fairly waterproof. On the other hand, if you want something that will be lightweight (under 1.5kg), three - four season and will last you for years then you really have to spend serious money or you will probably regret it down the line.
The Hillegberg Akto, is considered to be one of the best one man tents out there and is absolutely bomb proof. It's very expensive but is a lifetime's investment and over the years would be a good investment in my opinion.
I was lucky enough to get a Terra Nova Laser a couple of years ago when they were for sale on the Sports Direct website and, although it's not got the reputation of the Hilleberg, it is lighter (just about 1.1kg) and a very well designed tent which has withstood some pretty stormy nights on Dartmoor. It packs down small, pitches inner and outer at the same time and slots into the side of my rucksack.
Ultimately, I guess this is about value for money but I'd be suspicious about a tent that offers to do everything a specialist mountain tent can do but for a fraction of the price. Corners must have been cut somewhere and not just on the profit margin.
Martin