There's now (and never has been) a good way of contacting VM online as the company is digitally incompetent, as the shoddy chat service shows but presently there's a text/call back service if you text them on UK number 07533016422. I doubt they'd call you back on a foreign telephone number, but if you've still got your VM mobile and its working (roaming) then that will be acceptable. Because of the delays in response times and the time difference if you need to speak that might be inconvenient.
Failing that you can cancel in writing. Be aware that if you're still in a discounted contract for a fixed period, they'll want you to pay exorbitant exit fees. I can't see them coming to Canada to chase you if you refused, but non payment would cause problems if and when you returned to the UK.
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