to get any kind of treatment where opiates are prescribed you usually have to be registered as an addict by the local GP (General Practitioner, might be called a “community physician” in US English); but if you do not have British or European nationality whilst I think they will register you many services are charged for unless its an emergency. If you have dual nationality of UK or any EU country you can be treated on the NHS for free.
bear in mind though that going on any government database in a small country often means things like immigration visas etc get crosschecked, its relatively easy for authorities to locate any telecoms traffic.
Many years before Ed Snowden was even born some chaps who worked closely with GCHQ told me and my friend how, when and why this was done (all that has changed today is the technology). One of the things they said was that any discussion about class A drugs always got monitored, their advice was basically “don’t shout about it too loud if your doing them”. Nearly 3 decades later the exact same places are being used for the monitoring!
Added to which the govt and social attitudes towards foreigners have changed in the UK since the economic crisis, you may not want to draw attention to yourself too much if there are other reasons you wish to remain in the UK.
If you are genuinely unwell you will still get treatment but its harder to get opiates via “loose prescription” in the UK than in some areas of the USA.
If you have any chronic pain conditions that need opiate treatment they now send you to the “old peoples home” even if you are in your 20s or 30s (but that is only subsidised by the NHS if you are British/European) where the supplies are very closely monitored (to be fair they do give all the required other treatment for pain).
