Sorry for the daft question but what is commercial justification.
Not a daft question. In general, contracts cannot include penalty clauses for breach of contract - previously, it used to be the case that any fee charged for a breach had to represent a 'genuine pre-estimate of loss'. Of course, a minor breach of a parking contract does not cause Excel to incur £100 of losses, so these charges used to be deemed unenforceable penalties. The case of
ParkingEye vs Beavis in 2015 changed this in certain circumstances. In that case, Beavis overstayed the maximum stay in a free car park for shoppers at a retail park - the court decided that the fee charged by ParkingEye did not need to reflect their actual losses, because they had a legitimate interest in ensuring that people did not misuse the car park, denying shoppers the opportunity to park, and affecting the income of the businesses on the retail park. As such, the charge was commercially justifiable.
If you hadn't paid for your parking, then (notwithstanding potential issues with the signage as discussed), the charge would likely be enforceable - Excel have a clear legitimate interest in deterring non-payment, which is the predominant purpose of the £100 charge, and the charge may therefore be deemed commercially justifiable. However, you
did pay. Their attempt to justify the charge is to say that the tariff was not paid within 10 minutes of entry(or however long they expect payment within) - as discussed this time limit seems entirely arbitrary, and charging people £100 for not meeting this limit does not appear to serve any justifiable purpose.
It makes no difference to them.
It makes £100 worth of difference to them every time they catch someone out - their business model is contingent on issuing as many charges as possible.