Well when the term was coined, the key was two full width rooms each floor. Get out into the industrial and poorer areas especially in the North, Scotland and Wales and there were some one up, one down back to back terraces. Most of those went in slum clearances, but some survive...

Sure, the indoor bathroom/toilet was added later as it was a Victorian era innovation, but the second room on the ground floor was generally a back kitchen/diner. So you'll find a great many with a later kitchen extension (frequently flat roofed, and incredibly poorly, or not insulated) that permit separating kitchen and dining room, often adding an upstairs indoor bathroom/toilet for the first time. When those extra rooms were there from new build they weren't called 2+2s -- though they were clearly two bed houses derived from them.

UK estate agent standard for describing housing is number of bedrooms. Apparently we're meant to figure everything else from "two bed", "five bed". What else is in there is often quite varied and ludicrously (fraudulently) creative in estate agent blurb. Thank heavens for the relatively recent expectation of a floorplan and regulation limiting estate agent creativity. Until the 1980s or 1990s you almost never got a floorplan unless buying new, or exceptionally upmarket. :)