I think I can see the point that you are possibly making... that is that the socket (assumed to be a double or have a multiway adaptor plugged in) may be "overloaded".
A boiler will take perhaps 250 watts - say 1 amp and the heater may take another
12 amps or so - that's not more than 16A, and certainly not more than a ring with a 32A CCT breaker protecting it. The possible likely event is that the breaker might trip but that is not "dangerous" (is it ?) The same can happen at every other double socket in the house ! I suppose the boiler would not function if the circuit was tripped out so it might be cold.I really can't see why this is "dangerous" !
Nick