He said he was legally allowed to fly from the right seat
There is nothing in the law that says which seat a pilot must fly from however; the left hand seat in an aeroplane is traditionally the pilot seat which is why instruments and controls are designed to be accessable from that seat. As he is the only pilot, and you are a passenger, not a student, the pilot has certain responsibilities towards you, the aeroplane and third parties. Should he for any reason not be able to fulfil those responsibilities, then a court could question his judgement and consider him negligent.
In order to conduct these flights, the "approved organisation" should have an operations manual, and that should state quite clearly which seat the pilot will sit in!
An organisation intending to offer such flights as regular business activity is not considered to meet the condition of marginal activity. Also, flights organised with the sole intent to generate income for the organisation are not considered to be a marginal activity.
The CAA recommends that organisations conducting introductory flights ensure that the guidance is adopted within the organisation’s Operations Manual, or equivalent.
I asked to fly with a Flight Instructor but none was available
In which case I assuime they reduced the cost as they did not have to pay the pilot!