Ben, a warm welcome to the forums
I also like the Morrisons own-label Islay, it's great value as you say. Pretty much like it, and at a similar price of circa £15, can be found at Asda (a 'Pure' malt ie not from one 'Single' distillery like the Morrisons is).
Pay a quid or so more, worth it, for both the Sainsbury's Pure Malt (my favourite own label Islay) and Tesco (Single). M&S Islay is also good but at a 'whopping' £20 too rich for me
! Waitrose's offering was not so good for me, as a recent bottle I had I found some off-notes of rubber/bicycle inner tube and I suspect a fair bit of caramel/artificial colouring.
The thing is tho - they change all the time and you'll never know what they are, other than opinion and often conflicting rumour around here or perhaps some 'insider' knowledge ie we have discussed them here many times before. Indeed it was trying to find out what they were that led me here and after 2+ years I'm really none the wiser - all I know for sure is that they change from year to year and are now between £15-20, whereas a year or so ago they were £12-15.
I think, as SoI states above, they are much less likely to be Ardbeg or Lagavulin and more likely to be from Caol Ila or Bowmore, so from less 'in-vogue' distilleries that have high levels of both output and stocks.
My advice would be try a couple and if you find one you really like, wait til they go on offer and stock up - I bought a few Sainsbury's for £11 a bottle around a year or so ago