Yes I have done it but it is impossible to get £500 profit from £500 free bets.
You will make between £250-£300 profit.
It is possible to do the qualifying bets for no loss as you say at 2 different sites but you can`t cash out all the free bet for profit.
This is shown by the fact they offer you £100 cash or £200 free bet. If it were possible to to cash out £200 nobody would choose the cash would they?