Watching the youtube vid, it looked like the northbound tunnel has one solid white line, and the southbound has double whites.
I use the Birkenhead tunnel regularly and you're not supposed to overtake and the tunnel police enforce the limit strictly but sometimes people crawl along at 15 - and it's a lot longer than Blackwall - so it has to be done. Traffic in both directions too. £1.40 each way in the car, free if I'm on the bike
RH as a complete aside, do the police ticket people for driving without due care etc if they drive so slow that everyone gets frustrated?