This whole debate is, of course, an example of the philosophical puzzle of essentialism -- what is it that makes something the thing that it is; or what is its essence. This is a notoriously difficult problem in philosophical circles. Is something's essence just one quality (being born in Britain), or is it a combination of qualities (being born in Britain, speaking English, having lived most of one's life in Britain). And, if the latter, is it possible to remove some of those qualities, but for the thing still to retain its essence (can someone who speaks English, was born in Britain, but who has not lived here for most of their life qualify as British?). And, if you go on removing qualities, at what point does the thing lose its essence? The classic philosophical example is that of a ship which gradually, over time, has its planks replaced until, eventually, none of the original planks remain (the ship is only made up of replacement planks). Is this still the same ship, or has it lost its essence? (One might ask the same question of human beings, whose cells are continually replaced throughout their lives. Is an individual still the same person at the end of their life as they were at the start?)
My husband and I were discussing the British Social Attitudes survey at home. After some thought, my husband suggested that what made someone truly British was the fact that they considered themselves British. But I'm not convinced by this. Take someone who was born outside Britain, visited on holiday, and decided to stay because they loved the country so much and felt so much at home here. For these reasons, that person might consider themselves to be British. But I don't really think we'd want to say that that person actually was British just because they felt they were British -- at least, not until they had lived here for a numbers of years, could speak the language well, etc., etc.
Our Tesco shopping delivery interrupted our social attitudes discussion. While I was waiting for my husband to bring the last empty crate to the door, I struck up a conversation with the driver. What was our conversation about? The weather, of course. 'That's what it is to be truly British,' my husband quipped, after the driver had pulled away. 'Being able to talk to anyone at length about the weather!'