LHR Lounge. I would go Singapore, as you no doubt familiar with Air Canada lounges
Onboard I would take a row 9 seat - most feet room. Out of London they also usually offer Beef Cheeks, its delish.
You dont say where your onward destination is, if in SA you will need to clear immigration and customs, before rechecking your bag at the domestic transfer desks you can use the arrivals lounge. If international departure on star alliance you will be able to use the SAA departures lounge