The MBA thinks that it’s not unusual apart from its size, although it is "charismatic and rather cute" ! This may be one of the largest that they've heard of.
The species in question is Ligia oceanica (the sea slater) (see http://www.marlin.ac.uk/speciesinformation.php?speciesID=4745). It is one of the largest woodlice (oniscid isopods) in the UK and the only British members of this genus, normally growing up to 3cm in length. Generally found on the upper shore (at extreme high water and supralittoral). Apart from its large size key ID features include the large multifaceted eyes, the size of the antennae and in particular the flagellum with has at least 10 segments.