It's been a while since I've done that flight. When I did it, customs between Canada and Greenland was not required. I don't know if that's still the case. Kangerlussuaq (Sondestrom) is a common stop, though Aasiaat or Ilulissat are very pretty, and worth the extra distance for the scenery, if you can do it. Reykjavik is the common Iceland stop, but alternatively, I would highly recommend Vestmannaeyjar (island to the south) or Akureyri (at the end of a magnificent fjord to the north as picturesque alternatives to Reykjavik. In any of the three places you can walk to town. If you can stop in Vestmannaeyjar or Akureyri, budget a day for sightseeing. Vestmannaeyjar has an amazing volcanic eruption history, and you can walk it all, I have twice. I have never stopped in the Faroes, though I hear it's beautiful. We flew the Cessna 303 Reykjavik to Prestwick direct, no problem on one of the trips.
Alternative, and very scenic route would be Iceland, Faroe to Floro Norway, then Bergen, then Stavanger, Kristiansand, hop across to Denmark, then Sylt Germany, then to Belgium as you like (Bremen is a good stop). The scenery between Floro and Stavanger is magnificent. Lista Norway is also beautiful, with a super WW2 museum at the airport (lots of captured German equipment). The nearby town of Farsund is maritime quaint. From Belgium, back to the UK is easy. Aside from the UK, when I flew, customs was not a problem at all from/in between: Norway, Sweden, Denmark, Germany, Belgium, and countries south to Croatia.
I hope that helps....