I wouldn't refer to any of them as great 12s or really good 12s. Barritt wasn't great and very limited. Great defence but offered the minimal going forward. Catt and Flood both ok there but not first choice by any means at 12.
Manu previously did seem WC at 12 for a period, he played really well and did manage some great passes as well as some great lines.
T'eo played average for England, really nothing to shout about.
Flutey was great but not English.
Then there have been others but none I can think of that were great. Would an England 12 have ever made it into one of the top 5 12s in world rugby, struggling to think about many (post Greenwood) apart from Manu. Although I may be forgetting some.
Others who never made it there, sums up our problems of trying to find a top draw 12:
Ollie Barkley, Anthony Allen, Shane Geherty, Banahan, Hape, JTH, 12trees, Kyle Eastmond, Burgess (lol), Piers Francis, Vesty, Hipkiss and Noon played 12 as well I'm sure for England.
(think Burrell and Tompkins played 13 for England)
Wow some of those took some thinking.