Here's the NIC website where I've found it and yes, it is referring to the 314.1, sorry I can't remember all the regulations numbers. I took a better look at the 7671 amendment 3 and yes you're right, technically a shed or an outdoor is not a special location so that's my mistake. I was misled by the IET guide to the building regulations, which lists garages and sheds with the special locations (part 5.7, 2015 edition).
Still, I would never use a lawn mower on the RFC for the same reason I'd give a shed its own RCBO (or a combination of RCD and MCB, not just one of them) because, being used outside, is more prone to tripping due to water, moist and damaged cable.
As for why I was PAT testing in a house, it was a social accommodation and we've been asked for a certificate by the company running it.