You should bring whatever documentation you can with you to Canada.
British medical practice is at the same level as Canada. You should have no problem
getting your medication filled. If you have a doctor in mind (GP) then just call the office.
Yes, some GP’s are better than others. Some are not very enlightened in terms of ADD/ADHD
I think also it is important to have a psychiatrist that can follow you over. That is problematic
in Canada which has socialized medicine (similar to UK) and access is an issue.
I would do some research and find a psych who manages Adult ADD patients
Psych’s are best when you need to fine tune your meds. if yo have never had a proper assessment
in the UK then it might be a good idea to do one in Canada
Unfortunately Canada is similar to NHS in terms of long wait times for specialists.
Also it may be a hard time finding a GP in Quebec or parts of Ontario (including Toronto)
What I do when I move to a new area is call the charge nurse in ER at a community hospital.
Explain to them I am new to the area ..with your British accent that will work in your favour 
Other place is to go to the after-hours clinics and ask the staff
If you need more specific information post back
Good luck.
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