OP is ignoring the cultural factor here. Regardless of store presence, I doubt that Nintendo could ever be very popular in Britain.
Nintendo has carved a niche as the family-friendly console maker and most of itsbig franchises have a cute and cartoonish feel to them. If you look at the games that sell in Britain, they're all either very realistic simulation games like FIFA or Gran Turismo, or they're games with a gritty and "mature" aesthetic. Games with a cuter or more whimsical style just don't sell here (one of the things that makes Persona 5's sales in Britain so shocking is that this is an incredibly hard territory for animé-style game).
This based on purely anecdotal evidence, but I always get the impression that British children are expected to "grow out" of cartoons and other child-friendly things at a younger age when compared to American kids, and that the stigma of liking things aimed at children is stronger here too. I remember back in primary school, it was pretty much universally agreed by 8-year-olds that Nintendo was "for kids" and that this was definitely a bad thing. This was a while ago, but I don't get the impression that much has changed since then, and I see the same attitudes from my younger cousins.
We all know that Nintendo de-cutified a lot of its Japanese boxart in the 90s and early 00s in order to appeal to the American audience:
I think they need to do the same thing to an even greater degree if they want to be successful in Britain. Maybe they'd be one of the bigger manufacturers here if Link looked like this:
