While low-30s C might be hotter than average for us, it is cooler than the mid-30s C that is usual for Central Europe. Ventilated riding gear makes that doable in relative comfort, but I found my limit at 42 C near Lake Iseo in Northern Italy last summer - even with ventilated gear that was debilitating and very much "too hot to ride" for Northern English folk.
Ventilated gear might seem like an extravagance for the number of times you may use it in the UK, but it is not that much in the greater scheme of things & it does last a long time (as long as you don't "expand" !). I have a Hein Gericke jacket / IXS trousers & Mrs B has an IXS two piece suit, but other makes are available. It really needs to be mid-20s C minimum to use them & they have no water resistance, so we carry one piece oversuits for the occasional Alpine downpour.