I would take a look at here
Assistance Dogs (UK): A voluntary coalition of assistance dog organisations
They list the main assistant dogs organisations in the uk
There are some smaller charities that work in local areas,
I would speak to your local vets, speak to your dog warden etc as well as a local google search..
Hope this helps...
I'm extremely lucky with Ellie and Jones who took it upon themselves to do hypo warning duties..
Must admit though, sometimes Ellie can either use her talent for devious intentions or be a bit of a nag

She has been know to wake Les up on a red herring so she can sneak into bed with me
Oh and last night I didn't do my blood glucose test before going to bed... And no she wasn't going to allow me to go to sleep until I had done it, had her front paws on the bed wet nose in the face licking and licking, until I gave up and picked my meter up from the bedside draws to do a test at which point she went and settled down in her patch for the night...
We did think about training Ellie up to get her Official Assistance dog Licence but she's got a couple of minor glitches and also it wouldn't be fair on Jones when he couldn't come with us.. And sadly Jones doesn't have the confidence around people or strange places scars from his previous life before coming to us..