FCU/MCP selected altitude value is fed to the Mode S transponder in register BDS 4,0. Those NATS Mode S groundstations configured for Mode S enhanced surveillance extract that register data on a periodic basis, which is then displayed to controllers at units equipped with Mode S processing and display equipment - currently limited to LTC and MACC, although soon to be at ScATCC when move to new Prestwick centre.
Great stuff for preventing / detecting level busts! Other units in Europe aren't far behind. Strangely, IMHO the US seem to be a bit behind on this stuff.......