I've tried quite a few pushchairs!
What is best for you depends on what you're going to use it for and if you mainly walk/drive/use public transport.
Bugaboo bee (second hand for the price you want to spend) is brilliant, so light and narrow so excellent for public transport. The basket is a decent size and you can fold in one (although it can't be a tricky fold sometimes). It's good for older kids as the seat extends. A problem I have too is the wheels are rubbish off road, or indeed even on some of the pavements around earlsfield/Wandsworth common, the front wheels tend to get stuck on teeny kerbs...
Bugaboo cameleon (again second hand) is heavier and wider than the bee, but has better tyres for off road and some people like the fact you can get a proper carrycot for newborns. It's a 2 part fold so that might be annoying if you'll be folding it a lot, and difficult if you needed to fold for a bus etc.
I personally don't like quinnys, I don't like 3 wheelers generally and I find their baskets small - the buzz is also super wide on buses. But that's just my personal opinion!
I totally rate uppababy - vista is amazing, seat is nice and high up, massive basket, big weight limit etc, but it is also quite wide if you'll be getting buses a lot. Uppababy cruz is great, narrower and cheaper than the vista, but wheels are less 'all terrain' and I have heard bad things about the lack of suspension.
My personal favourite at the minute is the babyjogger city versa. I can't fault it, huuuuge basket, big weight limit, big seat, not too wide, amazing one handed fold... They do a GT version with bigger tyres, but haven't had any problems with the standard version.
There are numerous fb selling sites for pushchairs. I also recommend a website called best buggy - they have very comprehensive reviews of most pushchairs.