There is a national database of the grid ref's and lat/longs of speed cameras. If you find one that's not on the tom tom you let them know and they add it. Then when you're near it it shows on your tom tom map.
Also don't 100% rely on it as it got all but one last weekend on the A14 and didn't know anything about the new average speed cameras which I guess are going to be tricky for it.