The law states that if it is an exceptional circumstance affecting safety then compensation is not payable. The counter to this is to ask for airline statistics from the CAA under a FOI request to prove that it is exceptional. According to the Air Navigation Order (also Law) airlines have to report any technical defects or occurrences which affect or could affect safety undr the Mandatory Occurrence Reporting system. So by my reasoning when the flight is cancelled due to an exceptional safety event the airline is also required by law to report it to the CAA. You could ask the airline to prove that they have complied with the law in this respect. As an aside, the ANO also requires any person aware of such events to report them - so once you have been told I suppose that you should also report it. Get the airline to put it in writing and go direct to the CAA with your concerns that the airline has safety problems and ask if they are aware of them, and what they are doing about it.