Do you have a current AW profile with some positive feedback? (Doesn't have to be lots, just some)
If so, book a day room somewhere in central London that's mentioned as workable in the London hotels thread. You should be able to get a day room 10am-5pm for less than 100 quid. Then you'd only need a 1hr booking or 2 half hours to make a profit. Use free advertising as well such as Twitter, cracker.com, My Local Escorts.
Once you've got a good couple of days profit under your belt, use that to book a working room or a hotel for a few nights.
As Curvygal suggested you can also offer outcalls only for now and save as much cash as possible to pay for a working room. IME outcalls are less than 10% of your business if you can offer incalls as well though.