There are no clear exemptions for employers who wish to avoid paying NMW to actors. There are no categoric exemptions which apply to actors.
This is because the employment undertaken by actors (like any worker) is defined in employment law using very specific criteria and not by profession or industry.*
Click on any of these highlighted terms to view why these exemptions will not typically apply to actors.
In the meantime read this fantastic page from Businesslink.gov which clarifies the exemptions and defines many of the confusing employment law terms.
*except of course for share fishermen, who do have an exemption.