The assumption in the OP sounds legit, but it has a big flaw. In it, it is stated that the reason is lack of nostalgia due to computer gaming on machines like the Spectrum, C64 or BBC Micro.
If that was the reason, it would be true for pretty much all of europe, not just Great Britain. Germany for example was very heavely into the C64, Amstrad CPC (sold as Schneider CPC in Germany), Amiga and PC. France was very much into pretty much the same computers as germany, plus the Atari ST/TT lines. Heck, even today France and especially Germany are some of the biggest PC gaming markets out there. And the situation was similar in pretty much all of western europe, and eastern europe being behind the iron curtain would mostly not even know what a console is (they had however their own knock-offs of the western european computers) until 1990/1991 and the fall of the iron curtain and soviet union.
So while this all might have contributed to the overall weakness of Nintendo in Europe, it can hardly be the reason why the british gaming market is so different to the other european ones.