How do you think they start? I know that's not politically correct, but it's true. Nowadays I'm sure British people have lovely teeth, but in the 19th century and earlier when some would go on boats for months at a time with little supplies and even less hygeine, I'm sure their mouths were filled with rot. Proper dental care was not at the top of their list; survival was. However, since scurvy could eventually be fatal, many British sailors did suck on limes to get essiential vitamin-C.... hence another British stereotype/name began... limeys.