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ducted or piped, yeah. It seems like a less expensive option that running plumbing and buying new rads when you can get a new A2A system for £2k including the units within the room
plus added benefit of providing cooling in the summer if need be
gillies-
Air to air systems which can provide domestic hot water ought to be included under the BUS grant, but currently aren't, and aren't readily available in the UK in the first place.
I'd check their eligibility for the 'HUG2' grant first; if they're in a qualifying postcode then it's almost a given that they'd be able to claim for a totally free of charge A2W ASHP system without being means tested: https://www.gov.uk/apply-home-upgrade-grant
Failing that, I'd get a quote from a big energy provider for a A2W system, which will include all plumbing and rads. The total installed cost, after BUS grant, may be similar to an A2A heating system, with the bonus of ending up with a nice modern unvented hot water cylinder and cheap heat pump generated hot water.
ectoplasmosis
Do you mean ducted air-to-air or a unit in each room? An unusual choice (in the UK) either way.
And they're excluded from the Boiler Upgrade Scheme.