East Anglia, that's where I am from.
I have spoken to a few 350Z owners local to the area, and it seems that Marshalls Nissan in Bury St.Edmunds would be the dealer of choice.
This dealer knows the value of return business and has proven to provide a good to high level of standard.
Used to be family run before it was Marshalls, the majority of staff and mechanics stayed and continue to provide the service this garage was known for.
Personally, if it's a minor electrical issue I can fix myself, then I will at least attempt to fix it myself, but should I get stuck, will most likely take it to a dealer.
Remember, dealer charge by the hour, your own labour is free.
Whatever you do, don't take it to Glyn Hopkin, they have had bad reviews in the past.
Hope this helps