The LHBPA (John H is the distribution guy) carry stock of all "usual" bike polo equipment too, this includes polo balls of various colours, mallet shafts, heads, etc. We'll bring stuff courtside if you let people know the day before throw-ins.
It's all without a markup (balls: £3, shafts: £7-8, etc), so if you're ever in town and need equipment, just holler.
Harvey (owner of site above) is based in Cambridge, but supplies all of Europe with his balls, awesome guy.
http://bikepoloballs.co.uk/?