It really isn't that simple.

Every decision is a trade-off. The trade-off here is between on the one hand the savings from additional network reinforcement, the savings from reduction in aggregate Dx/Tx costs, and increasing the optimality of placement of generation wrt load; on the other hand, the cost of renewables generation being placed in areas with lower potential, the cost of increased price instability due to smaller markets, and of course, the switching costs. There is also the question of what incentives a zonal electricity market would actually provide to renewables developers.

With regards to the network reinforcement savings, it is worth noting a few things. A major obstacle to increasing network reinforcement is not the intrinsic investment cost, but inadequate and restrictive planning, which, the grid being a natural monopoly, results in artificially constrained connection supply (not out of malice but policy failure). Just as an illustration, the way DNOs currently determine whether to pay for flexibility services or upgrade the network is done on the basis of a _5 year_ calculation (ludicrously short!). The current waiting lists for new grid connections are on the order of a decade. Fundamentally, there is a short-sightedness in the planning system, and the long term is catching up.

As for the optimality of placement of assets, price signals already exist to reflect local needs -- there isn't exactly one single price for electricity for the whole of the UK (though it's a decent approximation). Transmission and distribution costs are baked into the settlement system. For grid constraints, both distribution and transmission use of system charges vary in space and time to reflect constraints (and flexibility services also introduce a local price signal, although I have earlier expressed skepticism of the procurement process).

If these price signals exist, why don't they cause renewables generation to become more distributed across the UK? The answer is that they probably do, but that grid losses are just smaller than the increased capacity factor of building in Scotland. Grid losses (both Dx and Tx) is on the order of 10%, and wind farms in Scotland will have a capacity factor about 30-40% greater.

Finally, to touch on the incentives question. The justification for pay-as-clear pricing (which is what you refer to as paying the highest price on the system) is actually to _incentivize_ the construction of cheaper, _renewable_ and nuclear energy. Sure, it doesn't especially disincentivize the construction of marginally-priced gas plants, but it doesn't incentivize it either. You could argue that maybe power companies are keeping this market structure to profit from their renewable assets instead of moving the whole grid to renewable, except for a simple fact: there is no monopoly on power generation in the UK.

Let me be clear: I am not actually arguing against zonal pricing. There are plenty of good arguments being made by people who have studied this more closely than me. What I'm fundamentally trying to do is provide a different perspective: that there is a lower-hanging fruit in the form of improving grid planning, a point which may be argued. But it is _not_ a simple problem with an obvious solution that's only being held back due to a conspiracy of energy suppliers.