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Not true - can be serviced by any VAT registered garage as long as it's done using genuine parts and to manufacturers spec
European Block Exemption...
https://www.theaa.com/driving-advice/service-repair/right-to-repair-campaign
The only thing I would say is that manufacturers have a goodwill budget should something go wrong outside of the warranty, if you've not had it serviced by a main dealer they won't pay for anything once they warranty has expired - this can be potentially costly, I've seen Audi pay £12k repairs on "goodwill" after the warranty has run out that they would have refused to pay if the vehicle didn't have full main dealer service history
Depends - if you have a manufacturer warranty then you'll have to stay main-dealer for servicing, if you don't have a warranty then a good indy makes more sense (generally).