Short answer: order before you fly and your Canadian number is working 24 hours before you land. From the UK the eSIM arrives by email as a QR code, or you collect a physical SIM at six Canadian airports. The postpaid plans include 1,000 international long-distance minutes to the UK, so calls home are covered without an app.
Brits are the third-largest group researching Canadian SIMs before travel, behind only Canada itself and India, and almost none of the advice out there is written for the UK situation: IEC working-holiday visas, university exchanges, and the two-year secondment. This is that page.
Why to sort it before Heathrow
Your Canadian bank account, your SIN appointment (which you need to be paid), your landlord's reference check and your employer's onboarding all send verification codes to a +1 Canadian mobile number. A +44 number will not receive most of them, and UK roaming packages in Canada are priced for a fortnight's holiday, not a year on an IEC visa.
The two ways from the UK
eSIM, by email
Order any time before flying; the QR code arrives within 24 hours. Scan it with your camera and the line activates 24 hours before the arrival date you give at checkout. Dial *#06# first: an EID number next to the IMEI means your phone supports eSIM. It must be unlocked, and UK carriers unlock for free on request.
Physical SIM, collected on landing
Toronto Pearson (Terminals 1 and 3), Ottawa, Calgary, Edmonton, Halifax and Winnipeg. Choose the airport at checkout, show your passport, and the line is active in about ten minutes.
What it costs, in the terms that matter from the UK
Under 90 days: prepaid from $25 CAD, roughly £14, no contract and no credit check. Staying longer, the postpaid plans are $30 CAD a month for 50 GB (Fido) or $40 for 100 GB (Rogers), about £17 and £22. Both include unlimited Canada-wide calling, unlimited international texting, and 1,000 international long-distance minutes to the UK, which no data-only travel eSIM offers. Ordering before arrival waives the $80 setup fee, and there is no credit check, which matters because you land with no Canadian credit history.
On an IEC visa or a study permit, the postpaid plan does one more thing a travel eSIM cannot: paid on time, it starts your Canadian credit history, which is the gate to a flat, a credit card and a car here.
The order to do things in
Order before you fly and give your exact arrival date at checkout, because activation is timed to it. Put the new Canadian number on your bank pre-registration and housing applications while you are still in the UK. Keep your UK SIM in the second slot for texts from home; the dual-SIM setup guide covers the details.
Common questions
Will my UK phone work in Canada?
If it is unlocked, yes. Canadian networks run on bands UK handsets already support.
Can I keep my UK number?
Yes. With an eSIM, the UK SIM stays in its slot for banking texts from home while the Canadian line carries calls and data.
What about calling home?
The 1,000 included minutes to the UK cover the phone calls, and unlimited international texting covers the rest. WhatsApp over data covers everything else.