I cant see a problem with it either. how many 240V extension leads do you see with black flex, white flex, orange flex etc. B&Q also sell ext leads with yellow arctic flex fitted with BS1363 plugs.
Possibly to do with cable length
Page 192 in the regs has notes on the extension cord however it is normal for the site to not supply the hookup cable, what gets plugged in is anyones guess.
That is where the regs contradict themselves, one says 25m maximum (not sure this is actually a reg or just advice) and another one says 25m+/-2m. But lets face it, if someone wants their caravan somewhere away from the power source they will cascade leads...
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