There are good places in central London for £10 including scan! Aperture in Rathbone Place is £8 for small scans (including processing) or £10 for big ones and they are very good. And Snappy Snaps in Wardour st are £10 for small scans and they do them in one hour! I use both all the time.

For postal use Bristol Cameras for cheap stuff £2.99 with scan! Or AG Photographic are great but a bit more expensive.