You could try Silverline http://www.silverlinewindows.co.uk/ windows in Bounds Green. A local Architect friend of ours uses them a lot. They do both aluminium and uPVC. Their website isnt great but we popped in to their workshop and received very good service. I personally dont like uPVC but agree with your point that in the loft, maintenance can be tricky so when we did our loft we used Velfac - http://velfac.co.uk/ which is the sister company of Velux where it is aluminium on the outside but wood finish on the inside. Velfac are mainly designed for use by architects and property developers so they aren't well set up for dealing with the public (although that might have changed since we did our loft 2 years ago). You also might find hat Velfac are hard to retro-fit in to a window space already in existence as their service relies on you to measure and spec the window sizes you want/need. Hope this helps.