I saw this frameset at Hunsley too.
It is almost certainly Raleigh built, not at Worksop though, that factory closed in 1980 - but Raleigh used the W prefix numbering system on the their"better"quality volume frames and bikes.
However, it could have been supplied as a trade bike to MBD and never logo'd up as a Raleigh.
Your hunch re the top tube lugs makes sense and makes it interesting - not exotic but interesting.
I think it's a little later than 1983 though (internal brake cable), is there a hole under that patch on the BB? If so thats where the riveted-on under bracket plastic cable guide goes.
Co-founder of a V-CC section, member South Eastern Road Club and owner of - 2005 Bianchi Reparto Corse (modern), 1985 Raleigh SBDU Pro Super, 1984 Raleigh LU Corsa, 1980 Allin Stan Butler Special, 1978 Ron Cooper, 1975 Bill Philbrook tourer, 2 x very early Roberts (mid/late 1960s), 1966 Raleigh Superbe Roadster, 1964 Allin Stan Butler Special Belgique, 1951 Hobbs of Barbican S/C, 1950 Hobbs of Barbican S/C, 1947 Hetchins Super Special, 1908 Centaur Featherweight. 1988 Specialized Stumpjumper.