There is still a cost. It's not simply the extra cable length, but also the extra labour this involves which is charged. (Excuse the pun.)
And as pointed out, no reputable contractor will utilise cable which you have supplied.
I certainly wouldn't entertain using cable supplied by a customer. They are always told "no chance" when they ask, as I cannot possibly guarantee its quality or even whether it is a legitimate product, let alone specs e.g. reactance to fire. (Dca, -s2, d2, a2 being the minimum permissible grade...
Far rmore usually near the 230V mark here. Can be down towards 220V in some places; up towards 250V in others. Also fluctuates at different times. It's a nominal voltage for a reason. It's never constant.
Assuming it's not a 2-way switch then it will make no difference to whether it's switched "on" or "off" which terminals the conductors are connected into.
In fairness very, very few existing DBs would be suitable, and they would also be unlikely to stock all different types/makes of protective device. Many brands don't have double pole switching RCBOs in a single module, which will rule many boards out.
In fairness I would suggest that it's very common for socket-outlets nowadays to be DP switching, although it was certainly much less common in the past.
If you can afford to pull more than 60A constantly through the night then you should probably just rapid charge your EV rather than fit a chargepoint, and heat your house by burning banknotes in an open fire.