https://www.theukrules.co.uk/news/homes-and-lifestyle/cars-splashing-pedestrians-law.html
In short, yes it is. And can lead to fixed penalty notices.
Prior: SEAT Ateca Xcellence Lux 1.5 TSI DSG MY19, VW Golf GTE PHEV DSG MY23
Current: Hyundai Ioniq 6 Ultimate
Next: we'll see what's available in 2028.