ASHP's aren't the most efficient things despite manufacturers claims, CoP's closer to 2 seem more the norm. Didn't they used to be called air assisted electric heaters? There should be some independent heat pump data coming out in a few months or so. Insulation and air tightness, as unsexy as they are, are the really important things to get right.
Going back to the OP, I reckon (evacuated tube) solar thermal to heat water is a good idea particularly for green rather than financial motives. Although the renewable heat incentive due next year covers it, at least in the consultation I think, so there could be an additional income from that depending how the scheme is implemented. Vented or unvented? – if the Navitron and builders forum don't help then find some local installers and have a chat.