Des Higgins writes

Let me put it another way; Blechnum is a common woodland fern that
looks just like the one in your picture; Christmas fern is not found
wild in the UK :-). The sterile fronds of Blechnum are mainly what
you see when you find it. The fertile ones presumeably come out
seasonally but I do not know when. Google it? There is lots of info,
once you know the species.

Not just season but also maturity of fern. I agree, these look like
Blechnum. Nice thing to have, even if it is relatively common,
--
Kay