We are doing a replump and thinking of putting in a gas combi boiler. We also have an eye on the future and so are planning to put in the wider pipes and low temperature radiators that a heat pump would need. Thought process is to be heat pump ready should we go that way in the future. We are currently a C2 so some upgrades still needed around the house before we are insulated enough.
Two questions:
Does it matter where we locate the combi boiler (we are thinking hot air press, upstairs) if we switch in the future?
Taking out your hot water cylinder & putting in a combi boiler with the plan to move to heat pump seems like a waste of time to me. You will need to install a hot water cylinder for the heat pump. Why not go for the heat pump now?
An air to water heat pump could bring you up to a B2.
I agree with sleeper, doing it bit by bit is a waste of time. get your insulation sorted before your heating system.
If you need to do a replumb consider using high-temperature rads but size them appropriately for a low-temperature heating system, stick in TRV's to control the output. For an air to water system I think one loop needs to be open but your plumber can advise on that. Just advise him of your plan to upgrade in the future.