I have not in details, but here it is, if anybody care to check it out: https://www.gov.uk/topic/working-sea/training-certification
I believe the UK is in line with EU and most of the rest of the world on this subject, which is STCW'10 compliance for all seafarers.
I don't see any specific OSV 10000 GT license, but there is special requirement for Workboats and Tugs below 500 GT and for Fishing vessels, but they are all STCW based.
Nor are there any 100 GT license, or licences for specific waters and trades, other then as specified in STCW Convention.
I obviously don't know, or pretend to understand, the present US system, but from my experience with US flag OSVs outside US waters, it did not induce much confidence by foreign Maritime Authorities when the "Master" of an OSV of 299 GRT could not read and write.
I understand that the standard has improved with the implementation of STCW by USCG for US vessels, incl. OSVs, working outside US inland waters.