To the contrary, The MCA does not accept American GMDSS certificates. I consider flying out to an MCA approved GMDSS course (which is not USCG approved) and living in a hotel for two weeks to be a significant expense, and barrier to getting an MCA CeC.
Nor is flying to the UK to prepare for, and take, the UKLAP exam, and the UK Master’s Orals an insignificant undertaking or expense, either. As I understand it, the MCA also requires 5 year renewals of some STCW courses for which the USCG does not require any renewal. The practical effect of that is that some original MCA approved STCW courses would have to be taken by most American mariners.
I admit that American mariners are victims of our incompetent USCG. May countries are on the MCA approved list of foreign GMDSS certificates, but not the US. The USCG has screwed American Mariners by approving US GMDSS courses that do not meet customary international standards. The American schools with for profit GMDSS courses have also screwed us. They could have easily included whatever small differences the MCA requires. But in reality, there is some, but no significant difference between the MCA and USCG two week GMDSS courses. The MCA could easily approve the US course for US mariners seeking a UK CeC, if it wanted to.
These are enough MCA created time and expense barriers to discourage most American mariners.