Jack of all trades. Part of the problem in the UK is the expectation that all of the passengers speak perfect English and can decipher a broad Gordie or Scots accent with no problem. Swiss (LX) do a bilingual announcement at Heathrow, but I can't recall anyone else doing one.
Frequent flyers know the system (they also know they need to get on early to get their carryon stored) and they know the worse thing that will happen if they try to get on before their turn is being told to wait. Non-frequent flyers don't know the system and just move with the crowd. Why not use that monitor hanging over the desk to tell the punters how boarding is going to happen rather than surprise eveyone at boarding time.