I've already started shopping for Autumn! The easiest way to update a wardrobe for colder weather is with knitwear. Cardigans can be worn open over summer tunics, or buttoned up over a t-shirt and maxi skirt. Knitted dress and tunics can be layered similarly over long sleeve tops, or under jackets. Choose colours which complement what you already own, so you can maximise your outfits, without spending too much.
A few days ago I found this knitted tunic in H&M for £24.99, (you can get 20% off a purchase if you sign up for their
newsletter). What I love most about it is it's versatility. You could wear it with a grey/black/green hijab and grey/black trousers, as well as jeans - me thinks this will fast become my Autumn favourite. It's also strangely reminiscent of this hijab-friendly Ralph Lauren coat from Fall 2008 RTW:

It looks a little fitted in this picture, but if you size up it's a nice loose fit, and hits just above the knee. H&M also happened to have a green crinkle shayla that would match perfectly (and doesn't need any ironing, yay!) for £5.99, but I couldn't find a pic unfortunately.

Green hijab £7.95, The Hijab Shop; black long sleeve top £7, Dorothy Perkins; knot and eyelet ballerina £25, Barratts; wire wrapped cuff £9, Accessorize; black ring slouch bag £25; Dorothy Perkins; jeans £65, French Connection.