Hi Lindsey,
I'm in Cobham, just over an hour's drive from you. And there are a number of other UK'ers on the forum too.
I do understand how stressful this must all seem for you right now, but honestly, please be reassured that diabetes is a treatable condition.
Like you, I had a fear of needles. In fact, that seems to be a very common experience on this forum; but
equally common is the fact that people manage to get over their fear...
Strange as it may sound, I got used to the idea of giving insulin shots by practicing on oranges. I'd draw up water into the syringe and then inject it into the orange. And I did it over and over and over until my hands got used to the idea.
I did find the first few insulin shots quite difficult, mostly because I was scared of hurting my cat. But I told myself, "My cat
needs me to do this for him." And once I'd clocked up a few successful shots (there were a couple of dodgy ones initially) my fear subsided and my confidence grew; and it very soon became part of our normal routine.
The insulin shot shouldn't hurt the cat. You will just be lifting up some loose skin and depositing the insulin underneath it. (You
won't be injecting into muscle.) Many of us give our cats their insulin shots while they're eating their food, and they often don't even notice (unless to think, 'why is that woman standing behind me....???)
Diabetic cats generally do best on a low carbohydrate wet/canned diet. Removing the dry food from Hamish's diet could reduce his blood glucose levels quite a bit. And since he's not on insulin yet this could be a good time to do that.
Once a cat is on insulin we recommend that the diet is only switched to lower carb if/when the caregiver has learned to test the cat's blood glucose at home, and can monitor the effect of the diet change (and reduce the insulin dose if necessary).
Fortunately, 'hometesting' (testing a cat's blood glucose at home) isn't really hard to learn, and, like the insulin shots, it shouldn't hurt the cat at all. It essentially involves pricking the outer edge of the cat's ear to get a tiny blood droplet; transferring that to a test strip in a glucose meter; waiting for the meter to count down and give the result; and rewarding the cat with a cuddle or a treat. (Or, you can crumble a few treats and test the cat's blood glucose while s/he is munching on those). Many cats actually come to enjoy test times because they know they will get a treat!

If you would like to learn to hometest we can help you with that.
Because it seems like Hamish's diabetes was steroid-induced, he may well be one of the lucky ones that goes into remission.
Incidentally, is Hamish a good traveller? If you think he could manage the journey to the RVC in London then you might consider trying to get him onto the RVC's remission trial. It would mean a few trips to the RVC's remission clinic, but you would be assured of good veterinary care, and would get free insulin and other supplies for a year... The link to the RVC clinic's Facebook page is here:
https://www.facebook.com/RVC.Diabetic.Remission.Clinic
Reassuring hugs to you,
Eliz