I think that any trial lesson has to be in a full Public C of A aircraft and most ex military jets are in the Permit to fly class. Although I have never offered such flights the CAA wrote to all ex military jet CAP 632 owners saying they were watching such offers very very carefully and would go after anybody who was quote "sailing close to the wind" erm good cliche that one !
I have a share in a JPmk5a and wont take anyone up for a jolly and always fly with my friend who also has a share. The main reason for this is concern over any liability in the event of an injury.
It could be argued that the passenger should be provided with all the latest serviced RAF kit, be of the right size for ejection, be of the right weight for ejection, not too light or heavy. Be medically fit, preferably with a certificate to show proof. etc etc etc. The propective passenger may want to dispense with the above for the chance of a ride, but should there be an incident I bet his/her lawyer wouldnt.
It aint worth it.