EU law says warranty is still valid if proper parts are used at a VAT registered garage. So, you can get a car serviced from new at a normal garage, and still enjoy the benefits of the warranty.

As unthrottled said, servicing outside the dealer network does lose any goodwill benefits later down the line.

Totally true but one other thing that gets forgotten. Whenever your car goes in for a service at the main dealer and they enter your registration/VIN on the system it tells them (or should tell them) of any updates/mods that need carrying out. Most of these are relatively minor but occationally one crops up that is more important e.g. ECU upgrade that extends turbo life on the wifes car, you don't get these when you go outside the dealer network. Having said that if you ring up and give then the reg/VIn they will normally check, they they paid for carrying out anywork by the manufacturer.

Another thing that can only be done at a main dealer on the wifes car is the annual paint/body check. Miss it and the 5 year paint, 12 year body warranties are capput. Its free when they do a service but I have heard of dealers charging upto £50 when customers have had a service elsewhere and only want the check carrying out and the book stamping, soon erroded any saving they made.