At one time Aussies and New Zealanders used to sell their campers for the air fare home outside Aussie House in the Aldwych
Moved up to York Way near Kings Cross last i knew and might be worth a go if you know what to look out for
Other than that try the classifieds in one of the Motorcaravan magazines or perhaps ask an old fart like me to keep an eye on the Camping and Cravanning monthly mag
Had several over the last 20 years and picked up a nice conversion on a Sherpa van that had only done about 16k for not too much cash along with a Toyota Hi-Ace and VW (worst of them all with the dearest spares)
Some nice ones out there if you look and get lucky, but some real dogs too