OK, I'm no expert! My information is largely based upon such things as conversations on trains with people who are in, or who are seeking to get into, the advertising industry. Disclaimer over, information follows:
While there may be a few people who work their way up from tea-boy to directing commercials, the general rule for getting any half-decent sort of job in the advertising industry is that you need a relevant university degree. 'Relevant', in this case, could include 'public relations', 'communications', 'media studies', 'art', 'design', 'advertising', etc, etc. (Basically anything dead easy and 'wishy-washy' - Oops, sorry, bias creeping in. My degree's in maths!)
Even for those with degrees, job placements can be hard to come by. There are plenty of 'opportunities' in London - indeed, the majority of jobs in this field seem to be in London - but the initial salaries offered often aren't high enough to support living in London.
As I've said, not 'expert' information but hopefully enough for other people to improve upon!
Chris