Well done Simon, I think it's a great idea to title a new post with UK in it so UKers know where to look specificially!
Could I just add - not all cats will eat Butchers. Some do, some don't - and if they don't, they have to eat something! Felix or Whiskas in jelly are great, not quite so low in carbs but absolutely fine. Tin or pouch equally good.
Try to keep kitties off dry foods of all kinds (unless you are not home-testing and are nervous about BGs dropping too fast), including "light" varieties - still very high in carbs. Meat though - pieces of chicken or ham - is great because it's pure protein and kitties will love it. Some people keep chopped cooked meat in the fridge to give their cats at "testing time", ie as a treat when a home-test has been successfully carried out.
Oh and newbie owners please note - treats are useful for yourself as well, eg chocolate/wine/ice-cream (delete as appropriate!)
Do please ask as many questions as you like, there aren't many of us here in the UK but we will always do our best to support you.
Best wishes
Diana (Surrey)