TUC are calling for an upper limit on work place temperature 
Bet any laws won’t include mobile workers, as if I give a toss anyhow, I used to work at a glassworks, ambient temp regularly got to 55C in summer, it’s the only place I’ve worked where management provided stout for your breaks, that is till the H&S started, FFS they tried banning smoking in the factory cos it was a fire hazard, DUH so the molten glass dripping everywhere wasn’t a problem?