If I recall correctly, you can apply now. You don't need to have a confirmed choice yet, just remember to change it when you actually firm if you need too - it just helps get the ball rolling if you do it now! Just apply as if you were going to your chosen university.
However, you will not get any finance until at least your first day of term, once you have enrolled in person. So, unfortunately you will need to find that £1000 in the meantime. I suggest talking to that university for support and more information regarding that deposit. It seems awfully high - is it a London based uni?