A 'G' reg aeroplane flown by the holder of a UK issued professional licence (whether JAR or National) is no problem in any country in the world - how do you think BA, Virgin, et al manage on international flights?
The problem comes when you want to fly an aeroplane registered in another JAA state - the theory is that your UK issued JAA licence is good in all member states, but the reality is that, to misquote George Orwell, some states are more equal than others. The French DGAC and, it would now appear, the Spanish have developed devious protectionist policies to prevent you from exercising the rightfully held privileges of your JAA licence in 'their' aeroplanes. The reality, of course, is that these 'National Authorities' are no more than puppets of their respective aviation industries.
So continue to fly your 'G' reg aeroplane with immunity - no matter which country's airspace you choose to enter.