I'd say Marks & Spencer as well.
I've had fittings there loads as i range from a 36G-38F. If you get a good fitter they will go out and find a selection for you to try on and see how they fit and often tell you the best way to put a bra on to get it to fit right, esp if, like me, you're more voluptuous :) They can pick the best bras to sit right, for example I have narrow shoulders so the straps don't naturally sit where they should.
It might be an idea if you go in with an idea of budget and the kind of thing you're after, maybe browse the shelves and pick up things you fancy , even if not in your size necessarily, so they can see the kind of things you like.
Bravissimo who cater for the more voluptuous, also free, also do fittings but there aren't as many of them about. There's a store finder here...
http://www.bravissimo.com/bravissimo/OurShops. aspx?language=en-GB
Sizes can change from shop to shop though so be careful. I was told in Bravissimo that M&S measure differently to them so to get measured again in there for their bras.