I'm currently using Mumford & Wood. Haven't installed yet, but looks very good quality so far.
Mumford & Wood are very good, spendy though? Can highly recommend London Box Sash Windows Ltd, beautifully detailed, great quality and come pre-finished.
Spendy, especially when you get the toughened glass, secure by design, etc. add-ons. I can't remember how much ours was, but I think it was in the region of £750 for 1200 x 800 casement in painted wood.
Mumford & Wood are very good, spendy though? Can highly recommend London Box Sash Windows Ltd, beautifully detailed, great quality and come pre-finished.
http://www.londonboxsash.com