Such a "Subjective" thing, not enough people (myself included) know ALL the Facts... younger fans go with what the TV has put before them in the last 30 years, older fans are usually "Stuck" on achievements (world titles), Historians can pull a few more worthy fighters into the mix - but still achievement based... "there really are DOZENS of men they Should/HAVE to considered!
Here is Historian, the great Gilbert Odd's list: Britain's Top 10 'middleweights' according to Gilbert Odd, (arguable)!
1 Randolph Turpin; 2. Jock McAvoy; 3. Len Harvey; 4. Ted 'Kid' Lewis; 5. Terry Downes; 6. Tommy Milligan; 7. Ernie Roderick; 8. Roland Todd; 9. Pat McAteer;10. Johnny Sullivan..
Considerations... Len Harvey was a L-HW and Britains best at that weight, Ted 'Kid' Lewis was a Welterweight and probably Britains best at the weight, likewise Ernie Roderick was a Welterweight and probably in the top 3 at that weight, Randolph Turpin was NOT better than Jock McAvoy - "hands down" and you have to include such fighters like Tommy Milligan, Jack Casey, Len Johnson (some would say Tops), Bert Gilroy, Ginger Sadd, Dick Turpin and tough guy Harry Scot...
There are a few lads who could put an "objective" list together (maybe Kev and Barry and KOJOE), say of the top 15 maybe 25, considering ALL the Facts...
but based on my reasearch I am confident in the Top 3, Jock McAvoy, Bert Gilroy, Len Johnson after that there can be and will be moving around, but I would say Milligan, Casey and Ginger Sadd would rate higher than most of the MW's of the last 40 years...
One man's educated/researched opinion.