Just wanted to update in case anybody is ever searching for this information, as it took quite a while for me to get a straight answer
It's been confirmed that my rabbits will not be granted access to Canada due to the fact that they have been vaccinated, current or previous vaccinations, with the combined myxomatosis and VHD vaccine. As this is the only vaccine for rabbits currently available in the UK, and vets here generally recommend it, I would imagine that anyone looking to relocate to Canada from the UK with their rabbits is going to have the same problem as me
They advised that we could keep having them tested until such time that the test shows that they have no antibodies of the vaccine remaining in their system, but a vet at the company who manufacture this vaccine has said it could be 5 or 10 years before a test would show up negative, and there is no way of guessing how long it would take. So it's basically a no if your rabbit has ever received this vaccine!