You’ve confused the flats ronib. Boris Johnson lived in Number 11, Sunak chose to live in Number 10, which he had previously lived in when he was Chancellor. He paid himself for new soft furnishings when he became PM, for which, good on him. I can’t find it in the public domain whether the annual parliamentary redecoration allowances were also spent or not, but I imagine he had already upgraded No. 10 as Chancellor, and was only out of it during Liz Truss’ short tenure when Kwasi Kwarteng moved in. I assume the Johnson horrors at No. 11 were removed under Liz Truss because the wallpaper was already peeling,, with Jeremy Hunt completing the job (not personally I would guess!).
There’s a bigger problem with Downing Street apparently, as it requires a structural refurbishment, which successive PMs have put off, preferring to paper over the cracks.