You'll be expected to join Unison if you become a police staff employee. It's certainly not compulsory by any means, but if negotiations are required, better the union who represent thousands of staff than a union that represent 3 or 4 pilots
And the quickest way to alienate yourself after gaining direct employment status, is to threaten your employer with unions
The onshore industry is fragmented, always has been and always will be. It'll never change, or if it was to change it would take a generation. Concentrate on what you can change which invariably means your little empire you work for now. Get directly employed and chip away at the machinery quietly. Educate the police in the art of aviation. If you can't or won't go for DE, then do what happened at PAS - it seems to have worked there
