PFCO / PAFW.
In order to fly for any commercial purpose A PILOT must hold a VALID Permission for Commercial Operation.
This is granted by the Civil Aviation Authority. Flying without the permission is classed as a criminal offence, as is any breach of the Air Navigation Order. Holding a current Permission for Aerial Work is the only way to get specialist insurance for commercial aerial work.
Bakehouse Aerial hold valid Permissions for Aerial Work, one covering congested areas and the other covering non-congested areas.
Never consider using a pilot that doesn't hold this permission. They're uninsured and working illegally. By hiring them you are putting yourself in an untenable legal position.