Dear Brit:
My two hibiscus were frost zapped two winters ago, but came back amazingly. But our five or six freezes this past winter did them in completely, and they have been torn out entirely. Although I'll miss their blooms, I've just had Oak Leaf Hollies installed in their place. This is a relatively new cultivar which looks great and shouldn't be bothered by the occasional freeze.
Considering where you are, might one of your hibiscus (hibisci?) have its feet in the Channel, and the other not?
I agree with Boomer's observation about the innate ability of you folks to grow anything! My late wife's cousin, now also gone a long time, retired to just outside of Lyme Regis from Yorkshire ages and ages ago. There he created his own arboretum with plants from all over the world, including a couple or three palm trees from Florida!
SWR