Asking chatgpt, in UK in late 80s, it would be 20-25% across all computers, depending on definition of "computer". A 1/4 of households is still high. It may have been higher in your neck of the woods.

> Did you live in the UK in the 1980s?

Yes.

> One specific model of ZX Spectrum

I do not believe this to be the case, that number appears to include the 128K and the Amstrad variants also.