Lynne is outside the church and notices a plane. The plane looks as though it is taking off and is very low. So low in fact that Lynne and the vicar must be shouting to each other. This means that the vicar must have some sort of public address system inside his church as those planes could be going over at any time.
The confusing thing here is you have to wonder why, considering she lives in Wimbledon, is Lynne visiting a church in Hounslow. Maybe she used to live in Hounslow and still regards the vicar highly. It is sensible to think that once George started bringing some money in, they decided to move away from the Heathrow flight path so to have a somewhat quieter life. Little did they know how frenetic life in Wimbledon would be.
The vicar must be making a joke but does this mean he doesn't fly? It may be that he is covering up his fear of flying by trying to make light of it. The truth is he is terrified of getting in an aeroplane and it haunts his dreams. It has also held him back in his religious career, having made him miss out on many ministerial conferences in Europe. How ironic then that his church is under the flight path.