Ive always used the powder coaters by Aldis in Exeter, www.1stchoicefinishes.co.uk/ But if you want a fancy metallic colour theres a company in the midlands that do a cracking job, but would need to find their card.
If you want them done in a timescale then check first whether they have a backlog. I get on well with the guys at ICS Cullompton who I trust with my TTR frames. See http://www.industrialcoatingsupplies.co.uk/
I'm over Ipplepen way I took over the old powder coaters here and relaunched as Spectrum Coatings South West. I have turned the place around brought new equipment but you more than welcome to pop in and have a chat.
Sorry for later reply I keep forgetting to check the forum.