Back in the day Nintendo were not really popular here at all. These days, with all the hype, and talk of Nintendo "saving the industry" and constant talk of Nintendo's games, and Nintendo's games alone (in regards to the 80s) you'd think otherwise, but its all really just marketing.
The month that Mario 3 was launched in the UK it was utterly, utterly crushed in sales by one of the Dizzy games (I think Magicland)
The computer market here was really strong, and established when the NES was released, NES got the reputation of being childish and mainly for kids, Master System gained more fans due to the popularity of the arcades in the UK during the 80s, Sega were a big name.
When the NES started turning up in UK stores, magazines were giving huge coverage to the 16-bit Amiga, and Atari ST computers, this was where the new gaming fans who had a lot of cash were going during this time in the UK, on top of that the UK magazines went absolutely crazy for the PC-Engine, which they had imported from Japan, virtually every magazine ran scores of PC-Engine reviews at the time, and people were clamouring to buy them, but after constant rumours over a period of years the system was never actually properly released here (there was just a crappy 3rd party attempt to bring it out with no advertising and little distribution). People in the UK really wanted the PC-Engine, not the NES, but sadly it never turned up.
NES later on gained a decent sized userbase of pre-pubescent boys, mainly on the back of Game Boy popularity (Game Boy was huge in the UK), and all the Nintendo themed cartoons that started turning up from the US (Mario, Zelda, Captain N).
Spectrum sold ~4 million (most of these were 1982-1985 though)
C64 sold ~4 million (most of these were mid to late 80s)
Amstrad sold ~2 million (mostly mid to late 80s)
Master System sold ~ 1.5 million (mostly late 80s and early 90s)
NES sold ~1 million (mostly late 80 and early 90s)