po pierwsze - nie pisz ze 'I want' (to jest niegrzecznie) lepiej ...I would like..
Dear Mrs X,
As you know, I shall be coming to the UK on the xxx of September. I am very interested in having the room and would be very grateful, if you could hold it for me until I arrive.
(p.s. Nie wiem czy bedzie chciala jakis deposit czy cos takiego - wtedy...If you require a deposit, please let me know). Thank you.
Yours sincerely,