I have to say I think this is a very harsh PCN, a money-grubbing PCN frankly.It is obvious that the OP did not intend to make a U-turn, but the road closure can only be seen once the turn has started. So what in hell was he suppose to do for heavens sake !!
I would take them all the way to London Tribunals if it were me, but it's the OP's money.