I've never had 'proper' sushi but have tried a fair few supermarket ones, which I've found to be much of a muchness. I recently bought one from either sainsburys or waitrose, can't remember which of those it was but it wasn't very nice and it went in the bin.
By far the best is Costco, I generally treat myself to their rather large sushi platter whenever we go, it does me two days.
Do you have a Sushi restaurant in Plymouth, pasta? We've only recently got what is considered to be a half-decent Chinese with, apparently, authentic food of which genre I do no know but I am presuming Cantonese. I've yet to try it. No Dim Sum though and certainly no Sushi down here :-(
We have a Japanese restaurant which was really good the last time I went. There's a fairly new sushi bar which is really good as you can watch them make it.
Any Lawson's convenience store, common across Japanese cities, does kick-bottom sushi. The British supermarket versions are pretty poor - rice too dry, tinned tuna paste etc.
But - I saw the seaweed sheets in Lidl! So you could experiment and make your own.