That's all good advice, but if you feel awkward about leaving you could just say "Oh, I didn't know prices had gone up, I'll come back with more money in a bit". I mean, the girl will be 99% sure it's bullshit but they do seem to be a bit friendlier if you tell a white lie instead of saying "Fucking hell love, I don't think so!".
Also, given the prices I'd say Soho's only worth it for a P&D, if you were to stay for an hour most girls charge at least £150, and some more than that, with less services on offer too. There's the odd exception (Crazy Angela being one of them apparently, though I've deliberately avoided her) but not many these days.
Btw, I've walked past 8 Greek Street on a couple of Saturday nights recently and seen that a "Nicole" is working there, but both times it's been late and they were closing up for the day, does anyone know if she's worth punting with?