Speaking as someone who enforces rules for driving, licencing and drugs offences daily.
I have no instruction for any exemption under the Road Traffic Act that allows users of ‘medically’ prescribed cannabis or any other drug to be allowed to retain a licence and drive…period. As stated on the NHS website, it is only issued to for those specific physical conditions which means automatically you have one, your licence is revoked. If you need reading glasses you have to update the DVLA, what do you think will be their assesment of Cannabis?
There are no exemptions, basically if you are prescribed ‘Medical’ cannabis your licence will be revoked on medical grounds, the Doctor issuing it also has a responsibility to inform the DVLA - not reliant on you at all to do it voluntarily.
All Police forces have a roadside test for drugs, if positive you will be arrested. Another sample will be taken at a Police station to confirm what drug is present and it can test MANY types of drug. You will then be subject to a blood test to measure the amount and prosecuted.
I have arrested someone for continuing to drive after they have been disqualified on medical grounds, due to the pain killers she was using. We are sent a letter by the DVLA to check every driver that has been disqualified, this includes home and workplace visits to ensure they are following the rules. If you drive while disqualified the sentances can be quite severe (see p126 of the Highway Code).
To the OP I suggest that you contact the DVLA with your question rather than ask on a public forum.