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One way to do a heat pump in a small flat would be a mini split air conditioner. The indoor unit(s) on those are normally mounted high up, so not far off heated ceilings.
Fitting the pipe runs and power for the internal units would be a ballache but yeah, it's possible. Getting the freeholder to agree to the external unit, locating it where it won't piss the neighbours off and sorting approval for it would be a pain in the ass I'd imagine too.
Howard-
sorting approval
I think you are allowed one external unit under permitted development, any more and you need planning. Ad-hoc installations of one per flat on a block is really ugly and likely to run into issues. It really needs a big system installing on the roof or something but then it's big works, freeholder run etc. Basically a nightmare.
jellybaby
One way to do a heat pump in a small flat would be a mini split air conditioner. The indoor unit(s) on those are normally mounted high up, so not far off heated ceilings.