Thanks for these helpful responses.
As for more details: we fly YVR-LHR-CAI in economy (arr. at LHR at T4).
We fly back CIA-LHR-CPT in first class (as part of a AONE4) and both the arrival and departure at LHR are T4. Specifically it will be for 5 days and, based on your points about cost, can be kept to 2 pieces, so cost-wise at GBP5.50 (according to the web site supplied) I can see it will be about CA$130.
With your point, about the left luggage being adjacent to the arrivals lounge, does that imply that one does not have to go out of the transit area to check in the luggage. A secondary question is does it make a difference if we check the single piece of luggage we take to CAI all the way through to CPT and then want to add to it with the 2 pieces we pick up from left luggage at LHR?