I'd agree more with JM123, £150 - £200 is in the ball park I think, don't forget, this is what you are going to charge A-N-other for a full days work using WFP.
Of course you can earn more, but for a days work that is pretty reasonable by and large.
London area, the South east in general I suppose you would be charging more.
If I were doing certain types of commercial work I would expect to be able to knock out £500+ in a day, but that would be priced work, a very different thing to perhaps another contractor wanting a hand for a day and wondering what you'd charge them.
As an example:
Is either Tosh or Squeaky came to me and asked me to help them out on a big job for a day and wanted to know how much I'd want for a day rate charge I'd probably tell them I'd want a minimum of £175 for a standard days work.
If it was me needing a hand I'd be expecting to pay them a similar fee for their help.
There is a difference between charging what you can potentially earn on a top day and what you will settle for as a sensible days earnings.
I have many days where my turnover isn't much over £120, and some bad days where I'm lucky to top £50
I'm not prepared to say what my good days are
Ian