Yeah, the law requires a the power output be below a prescribed maximum, not that you have a piece of paper stating that. Apparently the bits of paper can be handy to avoid having to have the bike tested, but I can’t see enough controls over the distribution of these certificates (and tying them to the presence of a restrictor kit) to believe that the police take them at all seriously anyway.
I’m pretty sure the electronic yamaha’s restrictor kits are a new ECU; you certainly used to be able to buy a bar-mounted switch for going between your restricted ECU and unrestricted one.