I think people do look at the ensemble output. It needs to be used with care though, and with multiple models.
Take today’s ECM ensemble. Even the top of the range only gets us to 24C yet it will almost certainly have hit 28C in London this afternoon.
GEFS had 23C as the median max, FWIW.
http://www.meteociel.fr/cartes_obs/gens_display.php?x=306&y=141&run=12&ville=Londres&runpara=0&type=3&ext=1
I blame the models' handling of sunny days, they do indeed undercook things for whatever reason.
If it's cloudy, though, or there's a bit of a breeze (not the light wind of today) then they tend to be more accurate in my experience.
As I said, a warm spell looks likely. A hot one - less so, at the moment.