It's actually not that huge an undertaking to move the Victoria plants to a greenhouse. Ours are all planted in pots in their pond so the pots get fished out of the pond and the whole thing moved like that. Messy, yes, but not too much of a problem.
It is also possible to simply heat the water. I just checked their website and they have them in the 'Waterlily House' and close this for the winter months.
http://www.kew.org/visit-kew-g...
