About 2cm long
No interactions present.
I think you have the correct species due to the three 'teeth' at the back end. I think the scientific name is spelt Idotea baltica.
Actually Idotea balthica is correct - I.baltica is a common misspelling.
I didn't say (originally) EXCELLENT pictures.
My new ID is NOT meant to subvert the correct one. But it is the only way to see the ID panel correctly formed.
Lat/Lng: 53.8103, -3.0569
OS grid ref: SD305354