Yes , seems pretty clear to me. Follow the money, speeding fines and congestion charge are both reduced income for the government if more people have motorcaravans rather than vans with windows.
Therefore make up the rules as we go along to suit.
If you would be kind enough to point me in the direction of the written acceptable means of compliance for making my van look like a motorcaravan. If you think I should just get over legally having to drive about at 50mph in a 180bhp 4 wheel drive with better handling, stopping and steering than many cars I hope you are stuck behind me on the A9 for hours.