Sadly, the cops (if they are involved) would ask you if you had posed the question to your mate regarding it being insured, or seen proof of insurance.
Sad thing is it may only have cost £15ish for day insurance, which is a small price to pay, hindsight is a wonderful thing.

I myself have towed a few vehicles without the vehicle being insured, though I wouldnt do that now.
Last time I done it was I was brining my convertible back up from Birmingham 7 years ago, the cops stopped us on the way into the services, but were brand new with us after we explained what was happening. I was told not to leave the service station without the car being on a dolly or trailer.