A house price has a lot to do with location as well, your son who lives in Bristol will be im guessing close to the cbd, close to shops, motorways , trainstation etc etc. Your other son who lives in a bigger house costing less will probably be a lot further out of the cbd meaning having to drive everywhere to get the same, a house close to brisbane CBD will cost you a million dollars so I'm guessing your son is 30kms out.
regrads to high prices in London, it's the capital of the world which brisbane and sydney are not