We do this, technically it is still your turnover, especially if the bacs payments are going into your account, the rental/subcontract fee is a cost to you only, unfortunately vat is based on turnover not profit, however if the clients were to pay the subcontractor directly he could then perhaps pay a commission fee back to you but obviously a lot of trust between the two of you for this to work, this would obviously weaken your turnover and keep you below the threshold
Graham