Passengers? Doubt that there's any passengers in air medical or police helicopters.
But from a cursory search it does seem as if there may not be regulations for helicopter flight crew and working personnel regarding fire-resistant suits (but there are for immersion suits https://www.caa.co.uk/publication/download/12736), but only recommendations from regulatory authorities and relevant associations, e.g.
https://pubmed.ncbi.nlm.nih.gov/10136597/
https://www.airambulancesuk.org/dress-code-air-ambulance-crew/
"The UK’s air ambulance charities do not receive any regular direct Government funding and so rely on public donations and fundraising partnerships to fund their lifesaving missions, including the cost vital pilot and Critical Care Team safety equipment such as flight suits and helmets." There's links on this site if you wish to support UK air ambulances.