YHZ also doesn\'t have very many flights to the US: NW flies there from DTW, CO has seasonal service from EWR, AC and AA have commuter flights from BOS, and I think Pan Am has commuter flights from PSM.
YHZ (and, to a lesser extent, YYT) can draw feed from the Atlantic provinces without diluting YUL to any significant extent.
As I said, shifting flights to YHZ is a desperation measure.
There will undoubtedly be seat sales before next July, and you should be able to get a fare below C$724.
BTW, the code for St. John's NL is YYT, not YHZ (Halifax NS).
Air Canada had sales on this year for most of the summer to St. John's (from Halifax) for $69-$79 (one-way pre-tax) so it may be cheaper for you to book 2 separate legs.
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GLA(thru LHR) to YHZ stay say 4/5 days onward to YYC for 3/4 days then return to YHZ for 10 days prior to return to GLA(thru LHR)...
I have no ideas for a cheap ticket, but I'm sure these are the things the experts will want to know.
Your best option would be to get the lowest fare between GLA and YHZ (It's showing $909 CAD now) and then get a seperate YHZ-YYC when they have a seat sale next year.