If in fact the destination of these non EU citizens is in fact Ryanair;
This matter has been raised before on these forums with the non EU Ryanair B737 crews from Yugoslavia on apparent Irish work permits then being long term based at Prestwick (UK)
The situation appears the same and as it appears that non EU crews are becoming 'ordinarily resident' in the UK both in nature of place of work and in case of where in fact taxable the situation should be addressed further...
Should recommend both Dept. Work/Pensions in UK should these non EU crews be seen to be operating 'ordinarily resident' in UK without correct UK work permits and also the Inland Revenue with regard to tax liability...as well as UK CAA