It is illegal for a private parking enforcement outfit to impose anything described as a ‘fine’. The rest depends on whether it was a 'private car park' owned by the mall developer, or by a consortium of shops. Or a council 'public car park'. If the former, it's a contractual penalty for breach of contract and is a civil matter that could, but probably won’t, go to a civil court. Last year members of the British Parking Association issued 1,800,000 'parking charge notices', commenced court proceedings against 845 drivers, actually took 49 to court and actually won 24 cases.

If it did go to court your argument would be that you were unaware you had entered a contract because the wording on the notice was too small. And anyway, you did not deprive the owners of the carpark of business. You could make a token offer of "reasonable compensation" of £10.

However, if it's a council car park it's probably a 'decriminalised parking offence', so you should pay and take the matter up with the parking adjudicator.