Nowt to do with RNR members in a crew.
As an RN or RNR Serviceman/Woman, past or present,
you can become a memberof the RNSA which is one of
twenty yacht clubs that can issue a warrant entitling the holder
to fly the blue ensign (undefaced) on any vessel they are aboard.
At a cost, as you say Jerry.
You could also join the Royal Yacht Squadron and fly the White Ensign
at amuch greatet cost
Ned