I would check with Chevrolet UK regarding Vauxhall servicing but as far as I'm aware you have to use Chevrolet Volt approved service centres and they are few and far between. The only Chevrolet service centre I know of for the Volt in the North of England and Scotland is in Edinburgh but you can check on the Chevrolet UK site.
This may be changing too as Chevrolet as a brand is being abandoned in Europe - I assume that some Ampera dealers will pick things up. Have a search on this forum as I think this has been asked elsewhere (not sure what the answer was though!).
I have spoken to my local Vauxhall dealer and they have assured me that they can service the Volt, the only difference would be that they won't not give you the enhanced service that you would get for an Ampera.
Honestly don't know.
Before I got the Volt I asked my local Ampera dealer if they would service it (only 11 miles form home) and was told as I stated above. When I picked up the Volt from Cambridge Chevy (40 miles from home) they were so good I would rather they looked after my car.
I live close to a Chevrolet & Vauxhall dealership, but the local service depot isn't authorised to carry out work on the Volt or Ampera. There is a service depot about 40 miles away which is part of the same group and is branded as Vauxhall and is authorised to work on the Ampera. When I asked them about servicing on my Volt, they said they can carry out the work, but it would invalidate my warranty!