It looks like Cordyline indivisa to me, but I have no idea whether those would be hardy in the UK.

Green varieties of Cordyline are pretty hardy in most places in the UK but the red and fancy varieties less so.

Green Varieties of Cordyline austalis, that is. The red varieties are usually hybrids with other species.
....
Well, I finally got a chance to do some research, and the C. indivisa and C. australis question is even more complicated than I imagined. Apparently, what I have been selling as C. indivisa all these years is actually C. australis, as is Advid's plant.
If anyone finds themselves with a Cordyline indivisa, though, that should be even hardier than C. australis:
http://www.palmsociety.org/members/english/chamaerops/003/003-18.shtml

....thanks to everyone who gave advice to identify my plant....