In the UK heat is rarely a problem, but can by humid or dry.
In Europe it can be hotter, but in places like the Alps you can often swing from one extreme to the other, on several occasions I have gone from just above freezing to 30c in just a few miles, often going up and down mountain passes sees the temp constantly fluctuate.
I had a cheap(ish) HG textile suit which was OK in mid temps, but probably only worked really well between 15 and 20c, the thermal liner worked down to about 10c, but meant over 15c started to get warm so I used to use a Microfleece if it got cold as it was far easier to put on / off than fitting the lining and doubled up as an evening jumper.
I always wear a thin base layer to help with wicking and to aid comfort (they always feel nice next to skin and stop bike gear sticking when hot) and think this is an important bit of bike kit in hot weather, handy as it can be washed and dried every night so saves carrying too many clothes.
I have just bought a new Halverson's suit with "Outlast Technology" which I thought was a gimmick, just a thermal liner - but to my surprise I have stayed warm enough down to 5-6c (warmer than old HG suit with liner in) on my commute to work with the jacket over my work shirt, and cool enough on my way home at 24-25c (as cool or cooler than my HG suit with the liners removed)
A few weeks back we had the very unusual situation of temps around 32-33c so I took the lining out of the trousers and left it in the jacket and went for a ride, trousers over bare skin (shorts) and jacket over base layer - to my surprise I did not get massively hot in the jacket, I sure would have preferred to be a bit cooler but I was nowhere near as uncomfortable as I would have been in my old jacket, and my legs were much worse feeling hot and sweaty despite having two less layers covering them.
With an Alps trip coming up I expect to test this kit in a wide variety of conditions, if it gets real hot I will remove the outlast again as I still cannot believe having a lining in helps - but from what I have read here maybe it makes some sense.