Which will still not stop the OP in using a miner to subsidize his heating, if the energy is cheap enough. At least what I see, is that you will get the bonus just for installing the system. I see no rules why you should not be able to use a secondary heat source also.
Which part?
Through the Boiler Upgrade Scheme, you could get a grant to cover part of the cost of replacing fossil fuel heating systems with a heat pump or biomass boiler
If it would have worked like that I could get a subsidy for using my computer to heat my room, hurry, I'm saving energy and the planet while I'm playing ARK.
The subsidies are for a more efficient system to be installed, an ASICs immersion system will generate just as much heat as cooling a normal space heater in that water, and you are still going to use electricity for powering up the miner, far more than a heat pump.
What people also have done is use their bitcoin miner to heat their pool.
And where is the energy saved, which is the aim of this program?