Density Altitude. While this is generally not a factor in the UK (low-lying, pretty close to ISA standard temp most of the time), it is a BIG factor when flying in hot and more importantly hot-and-high situations.
In Europe, pretty much the only places you'll encounter a real performance degradation would be southern Spain in summer, when temps can reach 45C+ in the shade. I once took off from Seville in a standard C172 on such a day - the ground roll was amazing
Luckily LEZL has a TORA of something like 11000 ft......
Once you get further south (incidentally to the area of your username!), then things become more complicated. Many strips in southern Africa are usable for t/o by SEPs in the early morning only during summer months, as you now have an elevation of 4000 to 6000 ft, high temps plus gravel/sand. Most definitely time to get out the POH and have a really good look at it! Same, of course, goes for the southwestern USA.
Quite a few 'lowlander' pilots have come to grief in the above areas.