Being English myself, my opinion is sure to be different from your own, but it might give you some ideas.
I don't agree that appearance is the essential factor, it is just one factor amongst many, such as body language, communications skills. The most important factor in getting a job is your apptitude to do the work. From experience, the job interview is largely to see whether the interviewer thinks you will 'fit in' the working environment.
Your appearance at a job interview sends a message about yourself and how you view the job. Obviously, if you don't make any effort to dress correctly for the interview, the potential employer knows that you aren't very motivated, so you are unlikely to get the job. In the other extreme, I've known someone turn up for an interview in a posh car, and nearly lost the job before the interview, because 'someone who drives a car like that will want more money than we can pay him'.
Another aspect to bear in mind is that in Britain and the States the 'dress-down' culture has become more and more prevalent in recent years. What started as 'dress down on fridays', when employees could leave the suit and home and go to work in jeans and a T-shirt, has extended to everyday, depending on the sector and individual companies. For example, most computer companies have a very easy dress-code.
In summary, appearance is just one of many factors taken in to account at a job interview. It is an important part of the 'first impression', together with self-confidence, communications, and body language, and in the short time-span of an interview, the first impression counts for a lot. However, in the all-out battle to get a job, appearance is a weapon to be used.
