Descol -
I think the previous learned posters have between them correctly covered all the likely flying or duty hours protections and the likely operational reasons involved in cases such as yours. I might add a few amateur observations. Please bear with - it's not simple.
I think the law requires the
true reasons to be recorded by the airline for later reference if or when questioned by a regulator. Such operational record (whether true or not) is otherwise unlikely to see the light of day as being 'commercial-in-confidence' or 'the truth that must be concealed from pesky claimants.'
Until then, a justifiably disappointed customer would get whatever public relations flaky flim-flam is ordered by management, in order to minimise legally required compensation costs. For example - this recent alleged event -
https://www.birminghammail.co.uk/new...y-out-25029532
I am told the UK's on-line small claims court process can work wonders if one's event is within it's jurisdiction. There may be European equivalents.
Just to add a bit of balance ... 
The subject of 'industrial limits' (a formal and usually less restrictive agreement between a specific airline and its pilots) as opposed to 'legal or regulatory limits' did not seem to be mentioned. Not all airlines have this humanitarian process. Some have it but only pay lip-service in practice.
I read regular complaints that airline managements view the flight time/flying duty period legal maxima as targets to be achieved (or even exceeded with 'creative' scheduling) rather than showing 'management weakness' by allowing a bit of sensible fat at the planning stage. One regular lulu a few years back was a four sector day planned to max duty, with the middle turn round (away from base) planned at about twenty minutes. That short turn round included a planned aircraft change, to one straight out of the maintenance hangar.
Unachievable in practice, and highly unsafe too, because you only have to miss one obscure switch in the wrong position to cause a lot of grief. I don't think the UK regulator gave a toss, but in other jurisdictions you can end up in jail.
I saw no reference to the many UK civil aviation regulatory and administrative problems that were carelessly created during the split from EASA. However, I see regular claims that the UK CAA is just not interested in,
or alternately, is now incapable of dealing with many of the often self-inflicted problems that regularly arise behind the more public face of civil aviation.
White Van Driver and Flying Clog mentioned 'Captain's Discretion' and the apparent protections for those who might think of using it's various legal stages.
Local interpretation of the intent and meaning of 'Captain's Discretion' can vary widely, both 'at the planning stage' and 'on the day' between an individual Captain and their airline, at both day to day scheduling and management (policy) levels. Even in 2022, differences in interpretation can quickly become hostile and hazardous to both flight safety and job security. For an airline to plan a schedule
on the assumption that the crew will go into discretion if there is any minor delay,
is a regulatory no-no.
The subject of Fatigue Risk Management and related schemes is another dodgy area.
Done properly it should prevent the more 'ambitious' scheduling excesses of so-minded airline management. Management usually views this process as jeopardising profit, so once again - glib lip service to the regulations and no regulatory interest.
"Also the good thing is we all know that there will never be any repercussion in Europe, where the above is taken very seriously by all operators, especially ultra low cost carriers."
For some interesting background to the danger areas of Discretion and Fatigue Management, I refer you to the judgment from an Employment Tribunal in 2016 - Ref 2400364/15. link -
https://www.balpa.org/About/Files/M-...-Judgment.aspx - Note - the litigant won hands down, conducting his case 'in person'. This was all the more remarkable because he was latterly obliged to run his own case, due apparently to lack of expected support at the crucial time. NB I think the UK was
'... in Europe, where ...' at that time.
A précis of some juicy bits ... He was charged with gross misconduct by Thomas Cook for refusing their 'reasonable management instruction' to use HIS command discretion. This was followed by a threat of dismissal for refusing to fly due to 'dishonestly stating' he was fatigued. However, TCA's own documents showed the predicted level of fatigue was equivalent in effectiveness to that of a drunk driver. Later in the case, despite senior managers stating on oath that the FDP was legal, it was determined and proven that the FDP had been wilfully falsified to 'make it fit', and to 'be efficient' by the airline.
The CAA was provided with proof of falsification by the operator and of duties being planned that would result in fatigue (both apparently criminal acts). The best response CAA senior managers could manage (via internal emails) was that the CAA would not answer questions that were only of 'academic interest' and that . Captain Simkins should simply 'get over it'.
Also discussed recently in these threads -
Wizz Air Crew Forced to Resign - and -
New book about Thomas Cook - Pulling Wings from Butterflies
The full story to date is told in Captain Simkins' 2020 book.
https://pullingwingsfrombutterflies.com Publication of the second volume is currently delayed while police investigate certain of his claims and supporting evidence therein, arising from the Tribunal. Don't hold your breath - usually in cases like this its "Move On. Nowt to see here." But maybe there's still a hope.
LFH