The Flying Log Book is a Personal Record, you can put in it whatever you want and call it whatever you like so long as you log those itemns required in Art 28.
The CAA is only concerned with ensuring that applicants for licences and ratings have the minimum hours considered necessary for issue, renewal or revalidation purposes. All guidance material is to assist in the identification of hours for those purposes.
Any form of checkout where you may or may not hire an aeroplane to the customer constitutes a "test". If you are there as an instructor with a FI rating you may claim it as instruction. There is no regulation that says you can't. An examiner logs his time as instruction without giving any instruction.
The FIEs at the seminar have no right to be selective and you have the right of appeal under regulation 6 of the Civil Aviation Act.