If you are in the London area:
London City is the "146 hub" of the UK because of the restrictions at the airport......Aer Lingus, AirFrance (franchise), British European, Swiss (Crossair), Lufthansa (CityLine) all operate 146/ARJ's from LCY to their respective hubs.
London Gatwick is good too, but the main user BA CityFlyer (or Eurogatwick as it is now!) is moving all its 146's to MAN and BHX to go to CitiExpress and will get 737's in return (LHR will get BHX's 319's)
London Heathrow has Manx (soon to move to Gatwick, it may have done already!) and Air France (franchise).
As for the rest of the UK, as I mentioned BA CitiExpress will boost their fleet with the ex-CityFlyer 146's, there's British European around the country and Buzz operates a fleet from Stansted!
Quite a selection for you! Hope that helps!