Are you happy with your bills currently ?
Let's say they managed to reduce your heating bills by 50%.
How long would that take to recover the £1700 extra cost of the pump ?
Putting water tanks in the loft is not great for the following reasons :
i) They weigh a ton, so the joists are probably going to need reinforcing, adding to the cost.
ii) Getting them up into the loft can be a big issue. How do you fit it through the hatch ?
iii) They have lots of pipework on them and potential for leaks. So the further you move them up in the house the more floors for the water to cascade through if there is a leak.
iv) Accessing them for maintenance is hard work.
Running radiators hot is not good either for heat pumps (they won't run hot anyway as the efficiency would be appalling) or combi boilers. For combi boilers to work as efficienctly as possible they really need to have as low a return temperature as possible, which is unlikely to be the case if the radiators are hot.
For me at the moment the greatest risk in remaining on gas is that the government will do something to increase the price of gas to try to force more people onto electric. This is unlikely because they are spineless (every time they try to tax people more or push up costs there is outrage and they backtrack) and also because the electrical infrastructure is not there to allow everyone to go electric all at once (they want people to move onto electric to achieve net zero, but not too quickly as the grid wouldn't be able to handle it).
The potential savings to be had from moving from gas to electric at the moment simply don't justify the costs and risks involved (for me anyway). If I was on oil or LPG it might be a different story.