Of course a doctor can prescribe whatever. But I don't think you can walk into a random GP and ask them to prescribe you some powerful drugs so you can withdraw from another drug habit. You'd probably best consulting a GP you see on a regular basis and coming clean with whats happened and your situation.
If they believe you then you'll probably find they maybe willing to come with a treatment plan that may involve anything from Clonazepam, Valium , Lyrica, Clonidine or Gabapentin. There are a wealth of options though your doctors obvious thoughts will be about your propensity to relapse.
At this point longer term options would be on the table to discuss such as methadone or suboxone/subutex (buprenorphine). These long term opiate replacement programs can be prescribed on a basis where you receive takeways almost straight away from a local chemist. You might have to double/triple dose until you've been on for a month and then they'll give you takeways.
In anycase it'll come down to how much you've been using, why you've been using, your history, age and other criteria. The aim of the game is to ensure that you housing and employment outcomes don't suffer. So I guess if you emphasis the whole job loss thing then maybe they'll come around.
But the best thing to do is be honest if there a good doctor GP that you have a history with.
