I've been searching, but haven't found an article I read where someone from the jobs help centre at the French Consulate in London (le Centre Charles Péguy), which mostly farms out small jobs to youngsters, explained that a lot of kids came over bright-eyed, but went back after a few weeks because they could only get low-paid work at first and the cost of living, especially accommodation, was too high for them. Some, like those you saw, may get decent jobs and have fun for a time, others less. But London certainly can be fun.