"Has anyone done this for 10/12mm figures if so how did you find it?"
Fiddly
No, really, it is - and of IMHO of questionable use - I think at this scale, at arm's length on-table, that you can 'fudge' things better with paintjobs & persuasion rather than resorting to a process which will drive you to distraction as you get frustrated with sharp implements, sticky superglue, misplaced cuts which ruin the head, dropped heads, and figure conversions which don't look quite right...
"converting 8th Army figures to Paras and giving Pendraken Brits in slouch hat helmets."
Have you got pic references of what you're trying for - I can't quite visualise why you're not just using Pendraken paras & infantry in helmets straight from the list