It’s interesting and a little amusing that when ACD was writing the Holmes stuff, the address he used for the detective didn’t exist; later on the address was created by knocking out a wall and connecting a couple of existing apartments. Er, I mean flats.
I lived in London yrs ago, and it’s been a while since visiting Sherlock Holmes’ site in Baker St London. You’re right apartments are traditionally called flats in the UK. IIRC, there was a bit of an explanation there how the site came to be, tho I can’t remember details.. as you know, Baker St is a major tourist attraction in London.