At least you are honest enough to give a link to the information at forestferns.co.uk, which correctly indicates that you just can't expect to be able to keep a Dicksonia squarrosa alive in your garden in Britain, except in a very few fortunate locations, or with considerable protection, because -4 is about the limit for them. There may well have been a frost of -6 last December where you kept them, but you evidently had the plants sufficiently well protected so that they didn't experience it. Giving them that protection without them going rotten is quite tricky.

The reason why most knowledgeable people who want a 6ft+ tree fern in their garden Britain rarely want a D. squarrosa, is that D. antarctica, D. fibrosa, D. sellowana, C. australis and C. smithii are all easier to keep alive.