First of all, ignore the coloured square bullshite on the PCN, as it is not mandated by the law or the regulations and is, in my view, unlawful intimidation to get you to pay.
Now, your journey. Which direction were you travelling through the tunnel? A toll fpor this tunnel is quite recent, and the signage may well not be up-to-scratch, and TfL have a legal obligation to erect adequat signage, and those signs must comply with regulations as well.
You say "the driver drove". This is an owner liability PCN, so are you the owner and driver ? Please explain how you are linked to the driver, as you may not have any role in the appeal process.