Named patient basis
No doctor wil prescribe this stuff to a person not his regular patient.
I am not 100 % sure how this works in the UK, but the usual way it works in Europe is that you go to your normal doctor, if you want a drug not on the local market, he will need to apply the medicines agency (in your case MHRA I guess, or maybe NHS) about a special permit (named patient basis). Once obtained you take your prescription (and your permit) and go to the local pharmacy, they will contact a special company handling such requests, who will then procure the product in question and supply it to them, whereafter you can pick it up.
Procedure might be even stricter since hydromorphone is a hard narcotic.