Paying a tow or cancellation charge

I arrived at my car as the clamper put the first clamp on. I offered to pay the fee to have the clamp released but he refused and said that it was too late and proceeded to put the other three clamps on my wheels whilst I watched. I rang the company who said I had no choice but to pay. Whilst I was speaking to them ( I was in the car ), the clamper proceeded to attach the tow bar. I was then charged £125 'clamp charge' and £250 'tow or cancellation charge'. Is this legal? Can the clampers put the tow bar on your car whilst you are present and then charge you to have it released?