> new builds will have to be built with heat pumps from (if memory serves) 2035.

England: from 24 March 2027

Scotland: since 1 April 2024.

The legislation doesn't mandate heat pumps specifically, it mandates >=75% lower carbon emissions than homes built to the 2013 standards, which effectively requires heat pumps, solar panels, and significantly improved insulation. Build and running costs aren't measured, it's quite possible new homes will cost more to buy and run, and could be smaller to compensate.

You remember "2035" because in 2021 the government announced it "wanted" to ban gas boilers entirety by that date: https://www.bbc.co.uk/news/business-58959045

... but as far as I know, never enacted legislation to ban their sale or fitting in existing houses. And that aspirational policy has been announced as dropped multiple times, at least since last July, most recently today