Hello and welcome!

I'm in the UK too. Waving to you from Surrey!
Well done, you, tor taking this boy on. That is incredibly kind of you, and it sounds like you've saved his life...
Feline diabetes can seem a bit overwhelming at first, there's a lot to take in. But please know that it is treatable, and with good care and just a little luck a diabetic cat can live as long, as happily, and as healthily as a non-diabetic. (My first diabetic lived for 12 and a half years with diabetes, and lived to be 20).
Since your kitty has been on steroids prior to the diabetes diagnosis it's highly possible that the steroids have tipped him over into diabetes. If that's the case, and if he's now off the steroids, he has a much higher than average chance of going into remission from his diabetes and becoming diet-controlled, either temporarily or permanently.
This section of the forum is just for welcoming and for saying 'hello', so are you able to post more about your boy on the main health forum? For example, was he on steroids for IBS or something like that? And we can chat more about diet, and other stuff... ...So do post on the main forum with more info.
Meanwhile, if you're up for a little reading you may want to take a look at the second link in my 'signature' at the bottom of this post. That has a lot of general info for UK newcomers.
Here's the link to the main forum:
http://www.felinediabetes.com/FDMB/forums/feline-health-the-main-forum.28/
Warm wishes,
Eliz