…eg- Blenheim mk 1,11,111,1V then the Bisley to mk V Blenheim.

Just to be politely pedantic, it’s Mk I, II, III, IV etc in Roman numerals.
Important to highlight as ‘II’ is two and ’11’ is eleven, for example there was both a Vampire Mk II (development fighter) and a Mk 11 (T11 Trainer).
The British Roman numeral mark system was phased out mid to late 1940s by the Arabic numeral system that is still in use, example Meteor Mk III became known as the Mk 3.