Originally Posted by
LondonElite
Why add a third country to a transatlantic trip? There is no concept of in-transit between Canada and the US so you need to collect your bags and clear Canadian customs when you get to YYZ. If you fly via a US gateway you don't have to do this (although admittedly you may have to change terminals).
This is only the case if your airline flies out of T3 or T1 and you arrive at the other terminal. Otherwise, as I just posted, the transfer is actually easier at YYZ on the outbound. And on the return, one must pick up bags at US customs and recheck them no matter what airport is used.
On Virgin you may get a massage or manicure. And its lounge at LHR is more fun than the rather dour AC/SK London Lounge. However, an American flying from an east coast city is likely to be getting a very affordable (>$2K) return fare in J on AC. I'm doing LHR later today for $1200 ex-BOS and this AC fare was also available ex-MIA for $100 more.
Of course, one final consideration: Virgin is not a STAR partner, AC is. So depends upon which FF program OP is intending to credit these flights to. Not sure if Virgin is affiliated with any FF program other than its own (and other Virgin programs) and even if it was, these would likely not be considered status-earning miles.