how difficult would it be to secure a training contract with an international UK firm with a 2:2 degree classification from a top Canadian University and excellent high school marks?

To be brutal, your chances will be very low.

UK law students with 2.1s and Firsts from the leading universities are struggling to get jobs, and many of those with TCs have just had their jobs deferred, which means recruitment for the coming years will be low.

High school marks will mean little as all law students come out with straight A's these days, and a 2.2 will put you bottom of the pile (in many law firms your application will simply be binned). Is your Canadian degree, a law degree? A JD from Osgoode or Toronto will significantly increase your chances. But if not, what do you offer to a UK law firm?