AGA appears to serve Kent
Scotrail needs edited to remove the London service
They should probably remove the Mull of Kintyre while they're at it, what with there being no railway south of Oban. The Caledonian Sleeper map could do with a bit more work, as it was clearly just copied from the Scotrail one, when the sleeper only reaches a few parts of Scotland.
I'm sure someone could put together maps of where different TOCs actually go, but I don't know if anyone outside NRE cares that much, either.
The original post asked "how could it be better designed", though. I was thinking that it could show special offers by area (as well as, or even instead of, by "train company"), meaning that it could highlight (say) East Anglia as you hover over it on the map, and clicking on it would bring up all of the offers relevant to it.
This assumes that the designers can find East Anglia is, of course.
I'd find this useful, as the special offers listed aren't, for the most part, TOC-specific ones.