Remember that if you are paid for giving instruction, it is aerial work. If you fly for an airline, you must declare any external aerial work you do to your employer and it will count as flying hours under your airline's FTL scheme. Your employer probably reserves the right to prevent or restrict you from doing aerial work outside the company roster if it would compromise your rostered duties due to FTL limits. The AEF pilots ran foul of this regulation and tried to argue that flying Air Cadets was "recreational". I don't know the outcome but I think they lost the arguement.
Of course if you wish to offer your services as an instructor for free, then that is a diferent matter. But then how could you recover the cost of renewal and seminar? Perhaps if the flying club were simply to reimburse these costs, it would not count as "hire or reward". Does anybody know the answer to this?
£500 for the seminar seems steep. I think the AOPA seminar is £229 inc VAT without acccomodation.