Do remember that standard 737s are lighter than their European counterparts, when either empty or full up.
On top of that Westjet's 737-700 fleet are less densely packed than Aer Lingus or Easyjet, thus increasing range capability.
The flight times are comparable for the Canaries and the East coast of Canada. My understanding is that some A319s do not like operating to the Canaries because they have performance issues, even from long runways. Easyjet comes more to mind, as their 156 seat A319s are very heavy! I recall being told about issues with bags being left behind on certain routes a few years ago.
In my humble opinion I think Aldergrove would sooner get Westjet, but it is not impossible for them to operate from the City. I imagine if fully loaded A320s and A321s can make it as far as Verona and Reus, a small 737-700 can make it 4 hours across the ocean in the summer. As mentioned before, a crucial factor would be if the engines are de-rated or not.