I'm sure Wicken Fen is pretty good for biodiversity, but I imagine it has these nice totals because it has a much high proportion of visiting specialists than anywhere else in Britain. It's also has a pretty long history of recording too.
It's worth noting that this is probably nothing near the likely actual total. For instance some other sites in Britain have over 2500 fungi recorded. See
here. These are noteworthy in that they have been regularly scoured by experts over a long period.