If you 'own' the car (eg...you have the V5) outright or have it oh HP, or
PCP etc, as far as I know, VAT garage with genuine parts would suffice. Although servicing at a Seat garage may get you some 'add on goodies.' Breakdown cover perhaps or 1 litre top up oil etc, depends on offer they may have.
However if you are like me, and have the car on lease, I just double checked my agreement recently thanks to this forum (which is with VWFS) it specifically says must be Seat Garage, despite the dealer telling me otherwise!
Eg...
9.4.1 make sure that the Vehicle is serviced and maintained
strictly in line with the manufacturer’s recommendations
and that scheduled servicing and necessary maintenance
is carried out only by authorised SEAT dealers