Hi Neil,
Greetings from another UK member.
If Beau is on Caninsulin at the moment, you can buy it online at the best price from Animed Direct provided you're ordering several vials. They charge for express delivery and cool packing it. If you're only using 2.5ml vials, it is probably much of a muchness pricewise to get the insulin from your vet, since ordering a single vial would rack up costs for the prescription and the carriage. Animed Direct are the cheapest I've found for Caninsulin U40 syringes, and they offer free delivery on them.
If you're looking for low carb foods, zooplus.co.uk have a great range to choose from. Also you can look online at the Happy Kitty Company. Both of them offer good quality low carb wet foods imported from Europe. Nature's Menu do low carb pouches, too (either online and, IIRC, Pets at Home). Have a look at brands like Bozita, Thrive, Catz Finefood, Grau, Granatapet, and Om Nom Nom. Some of the foods are more expensive than supermarket brands but they are much better quality (protein from quality meat sources, no padding with fillers). Cats don't tend to eat as much of them so they are more economical than the initial price tag might suggest.
WRT the diarrhoea, I'm glad to hear that Beau is recovering. My Saoirse had a really bad bout of it the week before last and she has been subsisting on home-cooked poached chicken and broth ever since. (She had a major flare-up of pancreatitis so I've got to be very careful now about what I feed her). I've just taken delivery of some Thrive Complete canned chicken breast in broth and it might help Beau if he needs something fairly bland till he recovers from his digestive upset. It contains basic vitamins, minerals and a taurine supplement so it is a complete food. Note that it's very low in carbohydrates, so you would need to be home testing before feeding it. (Note: If Beau is on Caninsulin, talk very seriously with your vet about reducing the dose when you start to cut over to low carb wet food. Beau could experience an astonishingly big drop in his blood sugar levels very quickly.)
You asked about Beau's always being hungry. With the diabetes, although he is eating plenty of food the cells in his body are actually starving. Cells need energy from glucose in order to properly utilise the nutrients from his food. Without sufficient insulin, the body can't use glucose properly so it mucks up cell metabolism. The cells signal that they haven't got enough food, so he eats more. As he moves towards regulation, the ravening hunger will subside and so will the unquenchable thirst.
