Do I just remove the existing UK file mapping by deleting off the etrex SD card, then installing the new files?
Yes.
You may prefer to copy the old file off the card so you can switch back more easily after your tour.
The HCx can use one map file only, which must be called gmapsupp.img, and located in the Garmin folder on the card.
More recent Garmins can use several map files, so what you download may have a different name.
There are utilities for merging different .img map files in to one,but it's probably easier to just download what you want from openstreetmap.nl.