It is always worth considering buying a ticket in WTP and use your miles to upgrade (MFU) to J.

For one way tickets, the fare obviously becomes expensive so LHR - YYZ on, say, 15 July is £2,148 + 12,500 BA Miles.

A return in July could be £1,103 + 25,000 BA Miles.

You could look at buying some BA Miles if you are short of a few.