I fly from England back home to Saskatchewan a lot and I've found my best options are through Calgary or Edmonton.
British Airways, KLM & Icelandair which codeshare with Westjet, or Westjet direct to London. If Winnipeg expands its Gatwick service it won't likely add much savings in $ or flight time for me since Winnipeg (& Toronto for that matter) isn't much less flight time than Alberta is (its 9 hours London to Calgary, 8 hrs London to Toronto).
Most of my flights from Europe to Saskatchewan have been either through Alberta or Minneapolis. I'd imagine most people flying Winnipeg - EU would be traveling through the same, plus Chicago and maybe down-east through Toronto if need be.
Keflavik is quite crowded when all its flights land and if operating on limited time to get through customs & get to the departure gate. The airport is expected to expand six-fold in the next 22 years to handle 25 million passengers though, at which point I'd imagine Iceland will be Winnipeg's next EU entrance point unless Schiphol or Frankfurt beats it to the line. It's the smaller western cities, that don't have/limited direct flights to Europe that IcelandAir will be targeting in the next decade or two.
In the future, Iceland Air as a hub would likely also be Saskatoon's return to non-stop flights to Europe.
https://www.youtube.com/watch?v=CYzJ1mK73IQ
https://icelandmag.is/article/keflavik-airport-be-expanded-six-fold-2040