Hi Jason,
I used Normandale in Daventry (so quite close to you) and they did an absolutely brilliant job. The contact at Normandale is a chap called Phil Ducker.
http://www.normandaleproducts.com/location.html
They do alot of motorsport work and are therefore very used to spraying glassfibre and aluminium vehicles. They did exactly what I wanted and even sprayed the inside of wheel arches and door pillars black for me.
I didn't do any prep work whatsoever other than stripping the car down and rebuilding after spraying. This helped keep the cost down (I can't remember the exact figure I paid but it was less than £2k).
I can't fault the work they did and they also offer a vehicle collect and return service (handy if the car is stripped down!). The sprayed panels were even returned wrapped in bubble wrap to avoid any damage!
Hope this helps
Simon