I think you'd be unwise to try it outside even in Devon, Jo. One or two people in California, where it's grown outside quite widely, say it will stand a very few degrees of frost, but the general view is that any frost will kill it - and wet British weather is probably just as bad for it. H. coccineus would be a better bet outside in a very mild garden in the South or South West of the UK, in sheltered well drained soil, or perhaps, if you can get hold of them, H. humilis or H. sanguineus.
Another option, of course, is to put it outside in the warmer drier months, and bring it in for the winter.