Hi Roger ......... welcome to the forum
Most UK fly lines and most trout lines are 'cold water lines' and can be affected on very hot days. As long as the water temperature is reasonable they shouldn't be too bad as long as you keep them in the water and wet. The affects are obviously more noticeable if you are doing some casting practice on grass, as there is no water to cool them down.
Tropical lines tend to be harder and stiffer to compensate for this, but then you will start to have problems with line memory at normal temperatures with a tropical line.
Best wishes
Mike