Seat belt anchorages and airbags are the easiest and most common checks.
Rebodies where the v5 has been changed would prevent the police complaining about airbags.
What is the law regarding public vs Private land. Assuming a supermarket car park is private land do the police have jurisdiction to check cars for example if somebody arrived at the meet and, after arrival, they put something on or took something off the car just while on show at the meet.