You would wait until you have 30 days left on your visit visa. Then you send in an application to extend, see
http://www.cic.gc.ca/english/visit/extend-stay.asp They will take about 3 months to even reply to that during which time you have implied status. You can state the reason for wanting to extend as that you are waiting for your PR to come through. Actually, since the London office is pretty fast, if you apply right away after coming to Canada, you have a good chance to have your PR by then.