Reverse bookings are crap imho, even if someone agrees to the date they may pull out for someone paying the full rate.
As for outcalls I think you may be asking too much for £150 in zone one and and I agree with fl0atab1lity an overnight will cost you way more than £500, that being said it may be just because I have a more expensive hotlist than average.
Personally I'd save my money if I were staying in Zone one and do incalls, you're only 20 minutes from virtually anyone in central London.