A couple of years back a “Tollkeeper” says “if your 3rd axle lifts up then you only pay 4 wheeler rate”, so i have always gone thro with tag up and sure enough only payed lower rate, untill yesterday that is, when tollkeeper says " £3.70 please youre 3 axles" i replied the centre one is lifted ,he says" dont matter 3.70",so now I am wondering have i been turning them over or did i get turned over! anyone know the definitive answer?
a few yars ago, u sed to go accross regular with a 26 tonne fridge rigid, i always lifted the axle and only paid the lower rate 
I always had 2 DART Tags for this but they stopped it when the prices changed about 2 years ago. Occasionally I have seen it get flagged as £2 since but then changes when the tag doesn’t lift the barrier.
when i was in my old 6 wheeler, used to go to kent and pay £3.70 and on the way back empty with the tag raised pay £2
the humber bridge is the same ive always paid the lower rate in a 6 wheeler with the tag raised
u got stiffed for £1.70 
crimbo is comming
It changed some time ago to the no of axles regardless lifted or not but sometimes it’s not always enforced 
strange thing the mob i work for
has 7.5 tonnersw with trailers all got stung a high rate
then suddendly it drop to £2
a result
I went across in a Drawbar and got charged double !
It seems multi axle goods catches everyone.
A few years ago I followed a bloke in a short wheelbase Transit van towing a a small compressor, he was charged £3.60 (2x £1.80) and I dragged my heavily loaded 8 wheeler through for £2.90
That was when trailers were charged at the same rate as the towing vehicle, but it seemed a right odd set up.