I think you may have a problem with the type of business that you have. Factors, like to have a tangible product with evidence of delivery etc. I use to have a PR company and was turned down by every factoring company that I approached because I was selling a service.
However, since I set up my jewellery business, I have used three such companies: HSBC who are excellent but I understand that they will now only look at you if your are projecting £300K+ turnover; Lloyds TSB Commercial Finance are pretty good too but both of these operate hefty minimum charges so you can end up with a very expensive facility.
I currently use Bibby's small business package which works on a flat fee basis with no minimums. However, the service is crap. Phone calls don't get returned and they will postpone the release of funds against an invoice for as long as they think they can get away with it! I have been using them for three or four months and have not really felt any benefit from the service. Unlike the other two they don't even credit check customers for you.
My opinion is that factoring is only useful if you have a business that is growing so quickly that you can't forecast an overdraft requirement accurately. If you are looking at it as a way of getting hold of some quick cash, I wouldn't bother.