belowradar
the club is charging you for a service, what they do with the money is up to them, they do not have to charge everything on a pro-rata basis. If their charges are not competitive they will lose custom.
The PPL FI could well be a ground instructor for which he can be paid, provided it does not relate directly to hours flown. How many schools charge you for ground instruction? I know some do, but most don't. The profit margin on a PPL is small so I really can't get excited about a club making £10 because the FI is a PPL holder. The customer is still getting good value for money.