I agree Jess - I find it really useful to order, then collect, because if I just go into the store, they often don't have my size, then I have to order it in, and as I live 20 miles from nearest store, that's a pain.! Also, one of our local M + S's is a small store with limited choice.
I do this in M & S, and often order 2 sizes, as they all come up different. I collect, then go and try on, and immediately return one I don't want. My rejects go into local store for sale, so others see stuff not usually available there. Also use Tesco Direct and Asda.