The Famicom was not available outside of Japan, so that is not comparable, and the Commodore 64 remained expensive in the UK due to tariffs. The Spectrum was the lowest priced color machine on the market in the UK by far, and it became very popular as a result. With a huge amount of software titles, It maintained good market share even though there were superior computers available--just like the Atari 2600 maintained market share for years after it was technically outdated here in the United States.
More importantly, for a generation of British youth, the Spectrum was the first computer they ever owned, and it was powerful enough and fun enough to be remembered fondly--which is why the Spectrum retro market is still strong over there.
A note on technical superiority. Yes, there were technically better machines available, but the Sinclair line of computers did way more with less. It's quite amazing that the speccy could do all that it did, considering its base bones innards. Its strength was its flexibility. It was well designed and well engineered (within the extreme cost limits imposed by Sir Clive).