You should probably start with the basic information about the city – the fact that it is a capital of Great Britain, its approximate population, geographical position and so on – however trivial it all may seem to be, your descriptive essay should be complete, so don’t omit any facts just because you think they are not all that interesting.
Then you should probably mention some landmarks of the city’s history – in fact, a descriptive essay doesn’t necessarily presuppose reciting the place’s past, but in case of London it would be really hard to describe it in its modern condition without paying due attention to what made it the way it is now. You’d want to mention that it dates back almost two thousand years and was founded as Londinium during the Roman rule. Other notable episodes of the city’s history are also worth mentioning, for example, the Great Fire and the Great Plague. However, don’t focus on them too much – after all, your main interest lies with the city in its modern state.
There is, however, a lot to be said about it as well. You will gain greater success if you specialize a little bit, because it is really hard to encompass all the factors in one little essay. For example, you may pay special attention to the ethnic composition of current London population and the size of different religious and cultural groups present there. Or you may follow a well-trodden path and elaborate upon famous historical buildings and places of interest, such as the Tower of London, Houses of Parliament, Buckingham Palace and others.
In addition, it is useful to keep in mind that it would be a futile attempt to cram all the information you have on London into one essay. It is hard even with simpler subjects, and in case of such a well-known and ancient city it would be utterly impossible.