Air conditioning is rare in the UK.
You may find some bland chain hotels with it but generally we are fine to just open a window.
The temperature in July is what it is. Some years it is in the low 30s and other years it may only reach the low 20s on a few days when it rains for the rest of the month.
You seem to be looking for problems on your holiday whereas to me it sounds like a wonderful adventure. Things will not always go to plan which is why we need to be flexible. It all adds to the experience and stories afterwards.
I have stayed in some lovely places but what I really remember is things like the complete stranger sharing an umbrella in a Singaporean rain storm, the quirky jazz bar recommended by the hotel receptionist to get out of the freezing cold night in Prague, the night time swim on a hot night in Melbourne, the fantastic Easter parade we just happened upon in Madrid, the tango dancing in the square in Aix en Provence, the goats cheese market in Marseille, playing football with the kids in Ghana, the Afghan restaurant we happened upon in Munich, …
in other words, the fun, unplanned unexpected rather than the guide book sights and meals.
And all of it was experienced with Type 1 diabetes.
France, Italy, Netherlands and UK are all very diverse, exciting, cosmopolitan countries. You will have a ball, especially if you go with a positive attitude.