As already pointed out here, replacing a head gasket is not an expensive or particularly difficult job but you don't specify whether there was any other engine damage. It's fairly unusual to replace a clutch within 3 years and also for a head gasket to blow. Does it have unusually high mileage? What size engine, has it been thrashed? What state is the rest of the car in? Just trying to understand why the nice AA man is suggesting you scrap it? You could try talking to Paul or Reg at Dulwich Garage for their advice on 0208 693 1442.