The old rule used to be that if the trailer exceeeded the towing vehicles weight, you were restricted to 40mph, hence why you can tow a ford granada on a trailer with a ford granada but you were restricted in the speed you could tow at.
That was a bit too simple for the law makers, so they introduced the new rules using MAW, and made the trailer entitlement seperate on cars instead of being automatically included
This caused more problems because with the new trailer entitlement being seperate, the MAW could be exceeded, so they brought in the C1+E to cover that and changed the old 7500kg entitlement down to 3500kg on the B licence, as the new C1 licence also covered the "big" vans
This then extended to D+E,D1+E and C+E, the only class that was "left alone" was A (motorbike) but to make everyone confused on that they used engine size, KSW, trailer width and length from rear of trailer to center of rear wheel
As a note on C and C1 licence it used to be that if you held a licence of a higher category then you automatically got the licence for the lower one, which means that if you drive a 72 coach on a C licence and recently passed your test, you cannot drive a 12 seater mini bus without taking another test
Hence why your everday bobby is just as confused as everybody else, but the specialist traffic cops supposidley are not?
and when they started on axle weights it really went to pot, hence why a single wheeled rear axle ambulance could not pick up an overweight casualty unless the paramedic got out and travelled behind in another vehicle (they are twin wheeled now so the axle weight can be increased)
total madness
If you are on the edge you are still taking up too much space 
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