I've used first hand both MyAlloys in Basingstoke and SilverWheels around Ringwood or something (Near Bournemouth).
I get on well with Marcus at Silverwheels. He powdercoated my AMG Aero II's for me in a very specific custom purple (it's a House of Kolor finish) and was very helpful the whole way. Very nice to deal with, helpful, nothing is a bother, always looking for solutions to difficult requests rather than the usual "hmmm bit tricky that mate.."
With powdercoat as always there is orange peel but once it was wet sanded (my buddy Nick @ DetailMyCar) did it for me they came up a treat.
For a normal finish MyAlloys in Basingtoke are good - I've put 12 wheels through them in the past.. 6 on my old Golf, my Dad has his done there too, and I paid for 2 of my Mums to be refurbished due to corrosion air leaks, and could not fault anything. The powdercoat was very thick and durable and never gave me any problems.
These are probably 13-14 months after being originally powdercoated. So very durable.
But since I've used them, they've gone very "corporate" expanding their business (fair play) into more units but losing that personal touch - hiring a receptionist who doesn't know her **** from her elbow (no offense to her) and making it impossible for me to actually have a conversation with the painters or the guy I had dealt with on multiple occasions previosuly
When I asked about custom colours she just blankly pointed at the "here's our available finishes" wall with the wheels on it. I asked to speak to someone else and she (basically) said no.
Shame really as it's an end to the business relationship I had with them.
I have a set of BBS RC's for winter use needing minor refurbishment and I'm going to have Marcus @ SilverWheels do them for me..
For £20 per wheel think of the cost and quality of the staff, materials, etc. All the decent finishes are £40-60 .. it's for a reason.
I guarantee if you do it on the cheap the finish will not be very good. A mate of mine had his wheels powdercoated by a cheaper company and they are pitted already. (around 14-15 months).
Do not do it cheap take it to a decent company. Silver Wheels Ltd :) They also offer 8 hour turnarounds if you are in a rush for them back.
They also did my BBS RC winters for me too..