@matt @starbreaker I suspect it's because big beige American boxes were expensive for us to import!
My anecdotal experience was that British people didn't fully switch to proper beige desktop PCs until the late '90s and early '00s. That roughly coincided with Microsoft Office becoming ubiquitous and the Internet getting cheap enough to use.
I remember electric typewriters and 16-bit machines made by Acorn, Amstrad & Commodore still being used for serious work when I was in primary school.