You haven't said where you are in the UK, nor how large your Cordylines are, but in principle, the idea is to gently pull all the growth up into the air and tie in that position, and then wrap the whole thing in horticultural fleece, including the trunk. Even this may not be enough in a very hard winter in colder regions of the country.
The fleece should be removed and the plant untied for mild periods, and replaced when it gets colder again.

Many thanks for your comments. I live in the Vale of York.