I went to Bicester last year and got a navy blue Prada city bag for £650! Such a bargain and I use for work every day. Definitely worth a trip to Bicester if you are close enough by.
If you want to get a general idea of what's out there in terms of designer, I would suggest starting on Netaporter. They only stock a selection and usually very trendy pieces but you can get a great idea of what designer you like, what style etc and go from there. Louis Vuitton not on there but you probably know already if that's what you want.
Stella McCartney bags are great value for money IMO, but not leather. Prada is a real class and they use saffiano leather and studs on the base, so v long lasting.
I posted a few £100-200 simple bag brands in my previous post