I’ve personally never tested it in court and never buy extended warranties so haven’t checked any wording. On two separate occasions I’ve spoken to Porsche UK. One because we fitted a Bosch battery to a Porsche Boxster and because it wasn’t their branding the customer kicked up a fuss come service time. The second was not fitting ‘N’ rated tyres to a Cayenne. Under block excemption these are non issues but both times I was told it would affect extended warranty.
Here’s one article which backs the theory up.
https://www.whatcar.com/advice/ownin...2016-what-car/
It’s important to check the policy contract for clauses that state or restrict where you can have the car serviced. Unlike manufacturer-provided new car warranties, extended warranties, both aftermarket and manufacturer-supplied ones, can legally stipulate that the car must be serviced by a franchised dealership. In practice, however, most providers are happy as long as the service centre is registered for VAT. Check with the warranty provider before having the car serviced if you are unsure.