I'd look to get an ASHP which heats the air directly rather than a water system. All the problems of water tanks and radiators go away. A new host of problems appear such as how to run the coolant to every room and where to put the heads. You also continue to use gas for your hot water and you aren't eligible for the government grant. You also can't plumb it into underfloor heating. On the plus side you'll have air-conditioning, smaller outside unit that could be wall mounted (maybe) and no need to change any plumbing. You can even fire up the boiler on those extra cold days if needed.