Comment by ccalloway

But for most of the time, the marginal cost of electricity is the same across the UK. The transmission network is not usually maxed out, and this means that generation anywhere can meet supply anywhere for minimal cost.

Consumers already pay per-unit electricity costs which are fixed over a billing period, regardless of whether gross prices exceeded the consumer price, or dropped below zero, for small intervals during that period. So it wouldn't really make any sense to charge different prices in Scotland and England, only for the periods when they actually diverged due to lack of transmission. And if this occasional discrepancy was averaged over a billing period, it would probably be much too small to really affect demand.