Cudz, your stretching my knowledge of the regs to the very limit.
I would say that hot water might be potable or 'wholesome' under the terms of the regulations, but because mains tap water shouldn't be tepid as that would potentially be unpaletable there is a requirement to prevent mixing. See here for the fluid categories
https://www.waterregsuk.co.uk/guidance/backflow-protection/backflow-protection/backflow-protection/what-is-a-fluid-cate/With my water treatment hat on the other reason I wouldn't want to mix in hot water is the residual disinfectant (chlorine or chloramines) that are there to suppress any microbiological contamination after treatment that might make the water unsafe. Heating the water will reduce this residual and will therefore mean that the water once cooled could easily become a viable media for biological growth, see legionella's etc.