Installer of ASHP here.....
ASHP can be very efficient so long as the heat pump AND the heating system are appropriately sized.
Chances are that you WILL need some/lots of radiator changes as well as the heat pump installation.
In answer to the specific questions asked later....
ASHP can go 'anywhere' - there is a noise so not outside a bedroom window - but most are quieter than a pressure jet oil burner so you may not feel that's an issue. The noise is more of a vibration than the 'roar' of a boiler.
If you need to run the pipework underground for any distance, the pre-insulated stuff can get reasonably expensive.
Yes, you'll need an ASHP-compatible hot water cylinder as you say - there aren't any combi-heatpumps that I know of.
I would get a local installer recommended by a manufacturer to survey the property - they should do that FOC and see what they say.
Hope this helps