The racing Rules of Sailing published by ISAF (not IYRU, Colin, for the last 20 years or so we have been Sailing not Yachting!) are very clear
Every boat of an ISAF International Class and Recognised Class shall carry on her spinnaker a sail number and, at international events, national letters denoting her national authority. They only need to be on the front side.
RYA Prescriptions apply these rules to all British owned boats.
So the rules apply, and the class, clubs and competitors are duty bound to enforce these rules. At present the only penalty allowed would be disqualification.
Note that a Race Committee would be obliged to protest a boat that does not have the apporpriate sail number, they cannot disqualify the boat without a hearing.
I would suggest that all SIs include the following:
“The penalty for breaking rule 77 may be less than disqualification”
In this way the Protest Committee can propose a suitable penalty – a generous contribution to the RNLI would seem appropriate.
I have always found that local sailmakers can supply sail numbers rapidly.