In the UK a psychiatric nurse cannot prescribe medication for you. A psychologist or psychotherapist cannot. Only Doctors who are licensed to do so can prescribe medications that are not available without said prescription (so listed as a POM). Pharmacists can issue POMs, but only to the holder of a prescription signed by a medical doctor who is licensed to do so.
I have a morphine prescription and have had for some time and I knew of one person who had an oxy script. But the NHS has tight restrictions on the prices of medicines it will permit (for good reason). So until oxy is available as a generic, it will only be available if you get a private prescription. Any doctor can offer private prescriptions, however, I wouldn't touch them. How are they in any way different to a drug dealer?
Private medicine has no place in the UK.
I tried some of his oxy and was very unimpressed. Obviously, I have quite the tolerance, but it was not worthy of the extra trouble acquiring it, when alternatives that are equally useful for your purposes are readily available.
As for diazepam..it is NOT a hypnotic (a sleeping pill). Sleeping pills are only indicated for up to 28 days use (max), because further use is actually counterproductive in treating insomnia.
Diazepam is indicated for the treatment of anxiety, however again, doctors are warned to avoid prescribing large doses, for long periods and to monitor patients very carefully. This is why we haven't landed ourselves in an opiate epidemic as the US did. NHS doctors are highly accountable for overprescribing.