the very grey area of a states competency as a member of icao and upholding the minimum requirments of the standard are the responsability of the governing states and not the operators.
The uk caa therefore as a government approved body would conduct an audit on the airline to ensure its competency to operate,the JAR and the FAR are a standard which countries have implemented to further improve the basic criteria set out by ICAO .
The only unfair advantage that an operator not so heavily restricted may enjoy is on the field of maintenance and operating procedures(eg:ftl) so they would only be comprimising safety for cost rather than lack of knowledge
the responsable as stated are the countries that allow the operators to perform 5th freedom flights under their current aviation policy.
if a non uk airline operates services other than scheduled flights from the uk then they are obliged to request a no objection from the uk operators for commerical purposes
However the approving authority will require proof of aircraft
registery,competence of airline to operate and proof of adequate insurance coverage to comply with that states min requirment
meet all those requirment and theirs no reason why you should not operate regardless of registration
the open attitude of european countries and to a certain extent the americas has made it easier for other operators to take over key markets-so its a government issue at heart- complain to your mp or better still brussels- meanwhile ill register my brand new 747 QLRF under a Zanzibar AOC