With all due respect, may I beg to differ.
All boats and crew must comply with the ISAF Regulation 20, Advertising Code. (Rule 80) Rule 80 may not be changed by National Authority prescriptions, the sailing instructions nor the class rules (Rule 86.1)
The default setting for all ISAF classes is as follows:
Advertising chosen by the Person in Charge may be displayed on hulls, spars and sails without restriction except on the spaces reserved for identification by Appendix G of the Racing Rules and under Regulations 20.5 & 20.9 (22.214.171.124).
However, ISAF Classes have the right to prescribe that no Advertising
may be displayed pursuant to Regulation 126.96.36.199, or in the alternative that only limited Advertising may be displayed (20.5.1)
For National Classes the National Authority which has recognized a National Class shall, on application by such National Class, decide whether Advertising shall be permitted or not and if so subject to limitations or not. (20.5.7)
All other classes shall not display any Advertising nor be required to
display any Event Advertising without the prior written agreement of ISAF, either in respect of a specific event, series of events or for a specific Class (20.6.4)
The Wayfarer is neither on the list of ISAF classes nor on the RYA list of National Classes.
In which case 20.6.4 applies. Unless UKWA has written permission from ISAF and Wayfarers in the UK may not be display advertising and events may not oblige them to display advertising unless they have written permission from ISAF (who presumably will charge a fee!). The situation in the Netherlands, Scandanavia, US and Canada is similar. The class may have become a National class in which case the National Authority may allow advertising. The local class association may have written permision, or the event may have written permission. Somehow I think that this is unlikely.
If the class wishes boats to be allowed to display advertising then it may be time to start contacting ISAF.
PS To my surprise the concepts of Category C and A have dissapeared from the latest version of Regulation 20.