Yes, at n=1, doing a boiler => heat pump replacement, in a cookie-cutter well-to-do house, will provide both cooling and reduced local carbon emissions.

But "well-to-do" is an important qualification, as your Guardian article notes. Because that expensive replacement work will likely be followed by higher utility bills in perpetuity.

Some well-to-do folks won't mind that. Others will. Less well-to-do folks will generally mind it more. Note that there are far more of the latter. And every one of them has the power to vote against the "heat-pump party".

There are other problems as you scale up - some noted in your cited article, some not. Britain isn't full of idle heat-pump factories and installation firms. You can subsidize - but the British Treasury is in iffy shape, and the pound sterling is no longer the world's reserve currency, to make that low-risk.

In theory (or your article), the right mix of competent policies and good judgement calls could make a British national heat-pump mostly-mandate work out well. But would a rational person, aware of the British government's very mixed track record over the past half-century or so, actually believe that they had the Right Stuff to do that?