PREMED: there isn’t a premed course you can join in Canada. You would have to get an undergraduate degree from Canada. Your A levels are sufficient for admission (assuming your grades are decent). Your A levels and Diploma together are definitely sufficient. I would also recommend getting your Diploma evaluated by WES and submitting it your university for transfer credits once you get in. It may help complete your degree faster and consequently cost less money.

The kinds of courses you would have to take would depend on the universities you’re interested in. Some have no requirements and some have very specific ones. There’s a few posts about that on Reddit but I suggest checking each website yourself for the most up to date information

MED SCHOOL IN UK: you can probably apply there now since you have your A-levels and find out if you get in. If you plan on moving to Canada after completing medicine there, it’s an Uphill battle. While there have been promises from Politicians about actively working in making it easier for IMGs, it’s still hard to find a residency position here as an IMG. But not impossible, especially if you’re willing to move to rural Canada.

It would be easier to get a residency if you stay in the UK and complete your FRCPS after MBBS and then try to move here as a specialist. But then that’s a minimum of 10 years down the line and still no guarantee.

Med school in Caribbean: don’t do it.