You can either find your own housing or arrange your housing through an agency. Arranging Housing means that you pick one of two rental agencies in London to locate a flat for you. Finding Housing means that you land in London, drop your luggage in temporary housing accommodations, and start looking at flats.
Arranged housing is simpler, but more expensive.
You are paying for the convenience of not having to locate the flat yourself. In addition, you are housed with hundreds of other American students - possibly not the immersion experience you were hoping for. On the other hand, when you find your own flat you can pick your neighbors and really become part of a community that you have chosen. Also, finding your own flat means that you can shop around for rent that fits your budget. But, you have to be willing to do some work when you arrive in London since you will be tromping all over to check out flats.