WestJet 737's are not that uber high density and will operate under ETOPS rules (and equipped) with at least 138 minutes capability for OEI.
They have 3 rows of 'plus' seats in the front of their 738 and has a config of 174Y v the 189Y maximum payload.
The 700 series only seat 134 pax.
ETOPS ''out of the box'' for the new Max is 180 minutes.
Plenty of alternates for a UK arrival if GLA wx is out - I am not sure if the GLA-Halifax is seasonal only in which case winter weather would not be an issue.
Astraeus crews used to do there and back (same crew) with their 737-700's operating LGW-Deer Lake (Newfoundland) with 148 pax.
Air Canada also operate small jets in to LHR on their shorter routes such as St John's. (A319 and 737)
Again I am not sure if WJ or AC operate any flights with a ''there and back crew'' on any of their shorter UK or Ireland routes to/from Canada.
My pal works at LHR for AC so I will ask him.
As HotelTango mentions flight times Eastbound will often be sustainability faster than the planned 6 hours+ when taking advantage of the prevailing upper winds.
Regards
PS
Monarch operated the very first UK airline ETOPS flight to/from MCO Orlando in 1988 with the 757 (via Bangor)