The block exemption regulations mean that you can get your car serviced anywhere (pretty much) as long as specified oils and OEM parts are used. This will keep the balance of manufacturers warranty intact up to 3 years. Pretty sure you’re out of manufacturers warranty on a Merc at 4 years (apart from bodywork).
I’m not saying use a main dealer for servicing but the one benefit is they will get the technical bulletins relating to your car for any future recalls which non (or wrong) franchises won’t. Merc will also definitely update the digital service record which could help at end of your PCP potentially with a bit more equity. Not sure if non (or wrong) franchises can do that.