I'm not at all knowledgable on the subject but I replied to someone the other day who asked about War exemptions regarding the Military Tribunal hearings that took place because I noticed that local newspapers have these Military Tribunal for war exemption cases in full and the official records themselves were later destroyed in 1921.
I don't know anything about anyone in the film industry being exempt (I saw your query in another thread and see that Alfred Hitchcock was exempt) I wonder if your grandfather had to go before a Military Tribunal? if so try the local newspapers. All those who claimed exemption were sent before a local tribunal, appointed by the local council, to have the sincerity of their claim assessed and it seems that local newspapers are now the only source of these records (only a few others survived in record offices) Maybe someone else will reply to you with regards to the film industry exemptions
My grandfathers were exempt because one was a foreman on Gladstone Docks and the other was a riveter at Cammell Lairds - my husbands grandfather was an Ag Lab and they were exempt.
Found this notice in the Caernarvon and Denbigh HeraldCounty of Caernarvon Notice to Farm Hands
The Military Service (Review of Exceptions Act 1917 empowers the Army Council to call up for Medical Examination men who have been rejected for Military Service or discharged . Men engaged in Agriculture whose work is of National Importance are excluded from the Act. Any men so engaged who recieve Stautory Order Army Form W 3479, should return the form to the Recruiting Officer with the certificate on the back of the notice filled in stating that he claims not to be able to have the notice sent to him on the ground that he is engaged in Agriculture and that his work is of National Importance
I've also come across other newspaper notices stating other various occupations as being automatically exempt from the Military Service Act(s)
In the newspaper I mentioned there are Military Tribunal cases in full including cases where the editor of the local newspaper tried to claim exemption for several of his employees, printers overseers, linotype operator and junior reporter, (The tribunal only granted conditional exemption to the overseer, the rest had three months and one month exemption) ...it's possible that your grandfathers employer claimed on his and others behalf, if he wasn't automatically exempt
There is more about the Military Tribunals in this National Archives pageHere
I'd like to know more about this subject too