In principle there is nothing wrong with UK registered airlines employing foreign nationals, be they European or from further afield, providing they have the required experience and qualifications. The world is a global market-place after all. However, many wannabes feel aggrieved that there is very little reciprocity between the UK and mainland Europe.
To argue that many Europeans speak good English and hence are able to apply for UK jobs only goes so far to justify what is happening. English is the main aviation language worldwide but the downside of this status is that every man and his dog speaks it. So if you are a pilot from mainland Europe and your second language is English you're sorted. Whereas, if you are British, what pray should be your second language? Take your pick of French, German, Spanish, Dutch, Italian and the list goes on.
In theory JAR sounds great but in practice I'm not sure any more!