As it is quite normal for the parts of Europe we tend to holiday in (Southern Germany / Austria / Northern Italy) to get into the mid-30s, I found a couple of things that make riding in such temperatures bearable. One is a bit more affordable than the other.

First off is hot weather riding gear - ventilated suits that are not water resistant in the slightest, but are basically an abrasive resistant mesh to keep the bugs out and hold armoured protectors in the usual places. My first was a Hein Gericke Air suit - I still use the jacket, but the trousers inexplicably shrunk over the winter a few years later & were replaced with IXS Air trousers. Mrs B got an IXS suit later. It has to be at least 22 C to use mine (so I don't use it much in the UK) & it allows reasonably comfortable riding into the mid-30s. I have an unlined shell water proof jacket & trousers that can be used in lower temps to block off the mesh & also provide waterproofing (after periods of very high temps come the inevitable thunder storms ! ). You still get a bit warm in slow traffic, but when you get going again the airflow through the suit evaporates off the perspiration and gives noticable cooling.

The other tip for riding in high temps is gain altitude - 1500 metres or more is best, but that is a bit of a challenge in the UK ! In 2019 we were staying on Lake Iseo in Northern Italy and it hit 43 C with high humidity. Even with the Air suits that was unbearable - in shorts & teeshirt and doing nothing it was unbearable ! IIRC Iseo is at around 200 metres, so we relocated North to Vipiteno at the Italian side of the Brenner - IIRC around 1200 meters - temps dropped to 34 C, but low humididty and a breeze made the temps bearable. Riding the Timmelsjoch Pass the next day (up to around 2400 M) was just pleasant.

Just back from "over there" & rode a couple of times in the mid-30s without too much discomfort - typically 130-170 mile days.

Nigel B