You're going to need to speak to the RYA Racing department who I'm sure will help you. There has been a precedent set with a UK racer running on a foreign licence, but I'm not sure how that would work the other way around.
In case you haven't seen it, here is the RYA's racing information page:
Here is the current UK rulebook too,
in first few pages there is contact details for the Racing Manager and others in the department, although I'm led to believe the current protocol is not to contact them directly but to route your inquiries through a UK club. As most of the marathon racing is Cowes/solent based, I'd have thought contacting the http://britishpowerboatracingclub.co.uk/
would be a good start.
Hope that helps.
How is the boat progressing?