It use to be the case that there were national scales; for instance, the Airport Operators Association use to have a scale. These days most airports seem to do their own thing so the only way to get a definitive answer is ring each and ask. Mostly it is done on weight per metric tonne rounded up to the next whole tonne but Luton, for instance, changed to a system, for commercial flights, of basing it on seating capacity (not no. of pax.) I don't know if this is still the case.
And let's call them aeroplanes or aircraft shall we?
