I’ve took a 15’9" high trailer through the Dartford Tunnel. They will stop you at the Peage if you are too high and tell you to use the right hand tunnel.
I don’t think an ASDA double decker will fit on the Woolwich Ferry and as for the Blackwall… just don’t go there.
I’m guessing that you are on an agency. What I would do is ask some of the other drivers when you book on or even ask if there is an approved/standard route for double deckers going to Dartford.
Safe trip.
W