Try your local glass supplier. There's usually one in within a mile or so in London. They can cut a sheet of glass and put a mirror backing on it. Most of them can deliver.
As long as you're not getting complicated stuff like beveled edges mirrors are basically float glass cut to size with a backing. Therefore they're surprisingly cheap.
Try your local glass supplier. There's usually one in within a mile or so in London. They can cut a sheet of glass and put a mirror backing on it. Most of them can deliver.