You are not looking at the wider picture Modelman. As a retired immigration officer I can assure you that there are hundreds of thousands of British passport holders, born in the UK, who speak either no English or very little. They fall into several groups. First, is those who were born in the UK of foreign parents, mainly from the Indian sub continent and West Africa, who went back with their parents or were sent back at a very young age and only returned to the UK when they were of an age to work. The doctor in my local surgery who I saw yesterday is one such person. Born in the UK she returned to Nigeria, went to school and graduated as a doctor there. She is now in the UK working. Her English pronunciation was appalling and if it was not for the fact that I spent a lot of time dealing with Nigerians both here in the UK and in West Africa, I would have had great difficulty understanding her. The second group who cannot speak proficient English are those who are brought up in closed communities eg religious Muslim and strictly orthodox Jewish sects.
Whilst I understand your concern at being questioned about your ability to speak English, we no longer live in a one culture society and I think you would agree that ensuring pilots speak a good level of English is something that cannot be taken for granted and checking their English language skills is a good thing.