C'mon man, you live near me. Great bus service, max payment on the oyster per day is what £3.80? that gets you into London from Sutton and a N44 and N213 home.
Nightbuses are smelly, come once an hour, are JAM-PACKED, go via EVERY SINGLE DISTRICT IN LONDON so the trip takes 4 hours*, and have puking people that want to lump you on them. And for the 213 you have to get to trafalgar square when hammered! From London (central) to Sutton a black-cab will ask for £60. £25 is the best you'll ever get - and that's from dodgy 'I won't rape you because you're a bloke' mini-cabs!
I stand by my figures to be honest .. although maybe I'd get a train into the city TBH .. not back out again at 3.15am .. and by the time you've got one of those daft 'bicycle' cart thingies you sit in to get to Trafalgar square, that's an extra £20 out of the pot again!
* This is an exageration ..