It really is a baffling case, Barnet Council objecting to the visitor voucher being used by someone who is visiting you and of course they don't live (as in their registered keeper address on the V5) locally. If the person who is visiting you has driven from, say, Birmingham then yes their car will be registered 100 miles away. That is the whole point of visitor vouchers.
If you buy visitor vouchers and then move two streets away, perhaps because you rent, and still live within the same zone, you aren't going to send back your vouchers for a refund and then buy new ones to link them to your new address.
The scope for the council to allege abuse will only be worse if you use virtual vouchers, so still carry on buying printed ones, which are easier to lend to neighbours who have run out and who return new ones to you the following week once they have some.