Confidence: I'm as sure as I can be.
Notes: Based on [Haywards seashore of Brit & Europe],which has only 2 UK Pisa sp's (as P.nodipes is Medit'n only) & [species.org] (2 also),& NBN gateway gives 2 Pisa subtaxa. Of the 2 Pisas only P. armata has a big spine at the back of the carapace as well as it's side spines, which despite the heavy sponge covering, show as lumps (the side ones slightly blackened at tips). see the species.org illustrations. Species.org gives size 33 (55) which I assume is female (male)