This was that I would pay a certain percentage of the lesson cost if for some reason I did not turn up without giving some notice.
Providing the percentage is not excessive and reflects the amortised costs of the use of the aircraft/instructor then it is unlikely that such a clause would be held to be unreasonable. The key here is the lack of notice regarding the cancellation as it prevents the flying school trying to re-offer the aircraft (for example to a student who is available at short notice).
The unfair contract terms legislation deals with clauses that attempt to remove or limit a consumers ordinary rights (for example excluding liability for negligence etc). Here the term of the contract simply requires you to give some notice if you decide not to honour your booking. It's not dissimilar to hotels or restaurants asking for a deposit/credit card number with a room booking/reservation and requiring any cancellation to be made by a certain time.