Obviously, what everyone has said is very sensible. I assume that you were thinking simply that it might pay dividends to walk into the offices and ask for advice, as opposed to sending out endless mails and getting no feedback. I can appreciate your point of view and, although as Forbes says extremely wisely, the bigger the agency, the more likely they are to consider you solely on the basis of reputation and credit, it is not beyond the realms of the possible that certain bigger agencies will take on untried clients if they possess the right look and attitude. But the basic truth is that the reason everyone has to spend many months, sometimes years, writing, emailing and chasing around to secure agents, is that most have a great many 'safeguards' in place to avoid being inundated with unsolicited applications for representation. It is almost certain if you do actually head to the offices of a major agency and get through the door, and are taken notice of even though you have no appointment, that you will not get further than the secretary. If you manage to get that far, I am not sure what you are then asking the agency to do for you - do you hand over your CV personally? (which guarantees you nothing more than sending it in the post does?). Demand to make an appointment? You will probably be told that only the agents can make the appointments on your behalf. And so on.
The bigger the agency, the more it is run like any other highly corporate business - and most highly corporate businesses do not play ready host to people who walk in off the street. In some cases, I am not even sure you will be able to get through the door. You'll be presented first with an intercom, asked who you are coming to meet, when your appointment is etc. and, if you have no prior arrangement, you'll find eventually that you can't proceed much further.
You may actually be much better off chasing an appointment by writing to agencies asking for precisely that - not representation, but just a preliminary meeting. Even then, they are unlikely to have the time to wish to respond. But, at the very least, armed with an introduction to the building, you will be able to get in!
It is worth pointing out that high level casting directors operate in exactly the same way - even when you are being cast by their office, you are unlikely to be standing in front of the principal casting director themselves - they have such things as assistants, and assistants to the assistants!
