There are several reasons which motivate people to be vegetarian:
Some people just don't like the taste of the meat, which is fair enough.
Some people are vegetarians because of their religion: for example, Seventh-day Adventists and Buddhists are encouraged to have a vegetarian diet.
Some people give up meat for health reason: they want to lose weight, or have arteries problems etc. I believe this is not a good reason to become a vegetarian, simply because some meats (chicken, turkey) are not very fatty, it's just the way they're cooked that make them unhealthy. Moreover, if people do not compensate the lack of proteins and vitamins with substitution products, they will inevitably get nutritional deficiencies, which can be a very serious problem. Doctors recommend to eat meat twice a week; so if people don't cook it in a pool of oil, eating meat is a healthy habit. It is the same for everything: if you consume any food in big quantity you'll get sick.
Some people are vegetarians for environmental concerns: it has been proved that producing meat necessitates a lot of energy and that the CO2 emissions are very high. I think this can be a good reason to give up meat, though I admit I wouldn't do it myself, I'd rather have an electric car ...
I believe that some teenagers become vegetarians just because it's a kind of fashion. It's a way of belonging to a group, to feel different and also probably because they can't bear the idea of eating cute fluffy animals. The problem is that many teenagers do not fully understand the consequences of such a diet, ending up less healthy than they were before becoming a vegetarian.
The last main reasons for being a vegetarian are ethical and moral concerns. Many people give up meat to protest against cruelty towards animals, and I think this is the difference between France and the United Kingdom. Indeed, the British farming system is completely different from the French system, and this difference has been accentuated by the World War II. Before the war, The United Kingdom was very industrialised, the farming industry was modern. On the other hand, France was a rural country and not modern at all. The war left both countries destroyed, with a disastrous economical situation. In 1947, the U.S government decided to help financially European countries that were affected by the war (Marshall plan, which was worth more than $13 billion in total). The United Kingdom received the biggest percentage of this help (approximately $3 billion) followed by France ($2.5 billion). An important part of this money was used to re-build the farming industry in both countries, but in a different way. Whilst the United Kingdom used it to turn the farming industry into farming factories in order to produce enough food to feed the whole population, the French government decided to modernise the existing agriculture, giving farmers subsidies to allow them to buy new machines. This made the French farming industry faster, more efficient, but it didn't lower the quality of the products, contrary to battery farming, where animals were often ill and badly treated. This difference still exists nowadays: battery farming is a very important industry in the UK, and it has provoked waves of protests since the seventies.
I think this is one of the reasons why there are so many vegetarians in the UK: people do not want to support an industry were animals are ill, fed with antibiotics and hormones to fatten them faster. Moreover, this industry produces poor quality meat, which has been revealed to be a threat to human health because of all the hormones/ antibiotics contained in the meat. In France, the battery farming exists, but it's much less spread: in the past people have been used to eat good quality meat that comes from the closest farm, therefore that generation has past on this habit. So, if the animals have been fed with good food and been able to run happily in the countryside for their whole life, I see no reason why we shouldn't eat them.