Take a look at aviationjobsearch.com
Alternatively, take a batch of CV's to Heathrow Terminal 2, up the stairs from the main shopping area, there's a big grey swing-door on your left. In the corridor's behind are the offices for places like Aviation Resource (who are currently looking for ramp agents, I believe), Servisair, etc. So start knocking on doors - the response might not be great, but I got an interview out of it.