Have a word with the guys at Hoylake Sea School. The Principal there is/was on the MCA Work boat committee that has been seeking to 'upskill' the wind farm crews and knows chapter and verse what the best route is. Indeed, suprise, suprise, his company offers all the relevant courses.
For my sins I run an offshore wind farm construction site in the German sector and you're right, no skipper of a PTV gets to work in the field unless they have a Master 200 ton COC and an unrestricted radio licence, as well as a valid ENG1 or equivalent, BOSIET/STCW95 Sea Survival, First Aid ticket, etc, etc.
The Uk sector isn't quite so tightly regulated yet (still loads of great skippers with RYA tickets) but it's fast heading in the same direction.
And yes, I have to say it, I've seen highly 'qualified' skippers (paperwork wise) who can't cut the mustard and many guys with just RYA Yachtmaster tickets who are superb....