[Subaqua]

I do not believe ASA will recognize RYA to the point that they will issue you an ASA certificate. I know that RYA will not recognize ASA to give you an RYA certificate, at least not without taking a course. I just checked to see about getting RYA certificate because I have ASA, and RYA said no.
As far as chartering or renting a boat, either course will be recognized as a certificate of competence and most companies will accept ASA or RYA.