Hi harveyjed, Cordylines grow much more successfully in the UK and Ireland than they did 12-15 years ago, at one time they would have been considered only suitable in areas which catch the gulf stream but now they are much more widely grown in all but the coldest and wind swept areas.
Your 8 footer is more likely to be more hardy than a smaller one fo say, 1 or 2ft but I think it's still wise to choose a site whare it receives good sunlight and shelter from cold north/east winds.
Where ever you decide to site it, it may be worth your while giving in winter protection (as many do) at least for the first couple of winters, one way of doing this is tieing up the leaves and covering with a horticultral fleece until all danger of frost has passed.
The Cordyline with the plain green leaves C. australis is more hardy than the ones with the red or stipey leaves.
Good luck with it. Tbird+