About preparation
Of course, I advise you to arrive in London with a place to sleep ! Some of students arrive this year without any accomodation and had to rent a room in hotels, which is not the cheapest way to live in london ! The BDE may offer you a solution for renting flat with friends, it may be a good solution !
About flats
Be aware that, in London, owners can rent you dirty flats. There are some laws for determining "liveable flats" but you can find flats without bathrooms or broken WC !
Try to live with english roommates or other foreigners. I lived with an english woman and then with an australian couple, that's a way to improve "freely" your english daily !
About agences
My first piece of advice is to avoid "City and Cities" because they are renting flats that are not "liveable" ! Some of students had to sleep in the same room with only one bed for 4 students !
You could find your flat with "Blix Properties", their flats are not cheapest but better than others ! That's the better deal I found in London for accomodation. You will have to know that nearest your flat is from Central London, highest is the price of the rental!