According to Good Food Institute (which is a plant-based food lobbying group), 35% of UK households purchased plant-based milk at least once during 2023 and 33% of UK households bought plant-based meat alternatives at least once during 2023.
https://gfieurope.org/blog/plant-based-meat-and-milk-are-now...
For a less biased source, a 2022 ipsos poll found that 48% of the UK uses alternative milk and 58% " use at least one plant-based meat alternative in their diet".
I think things dropped a bit since then due to cost of living crisis.
https://www.ipsos.com/en-uk/almost-half-uk-adults-set-cut-in...