Been driving for 14 years, and used to drive long wheel base vans although I don't have any enhanced driving quals. I can't comment on the highway code advise as I genuinely don't know.
Personally if I was in this OPs situation I would do exactly as above, hazards on and block the road - it might seem like driving like a d*ck but smaller vehicles will have to comply due to your size and larger vehicle drivers (vans, lorries etc) will understand and give way.
Be confident, be forceful and stay calm but most importantly be observant - the number of times I've had to swing right to take a tight left turn (indicating left) and had some muppet in a small car/bike/motorbike etc try and cut up the inside was too may times to count.
People might get wound up but there's not much other choice, a friendly wave to say thanks afterwards soothes a lot of ruffled feathers.