For galvanising services in the south east try this http://www.medgalv.co.uk/
A cracking service. They collect from you using a hiab lorry, galvanise it and deliver it back to you. You then pay by the kilo.
I had a combi trailer dipped a few years ago now and it cost me about £70 inc vat. Mate recently did his trailer for under a hundred quid.
I am shortly to start building a trailer for my Avon SR5.45 to replace a crappy home made one. Will keep you updated. Plans should be complete shortly. Will try and post them.