UK businesses claim that reducing the subsidy they get for gas would make it difficult for them to invest in using less gas.

That smells to me like bullshit. Cut subsidy for business, or raise taxes, and they will claim that they need that money to (1) invest in decarbonizing and (2) invest in innovation. Or whatever thing they now want businesses to invest in. But when they get that money, they are likely to do something else with it.

Instead of subsidizing their fossil fuel bills, and hoping they will use that to invest in energy efficiency, how about offering businesses loans specifically to invest in energy efficiency?