Good question - but has it been answered already?
Each local authority has their plans, and the budgets. They already have worked out how to allocate the budget against each portion of a bidder's bid. In effect, each plan must determine (for each exchange area) just how much of the budget is coming from the bidder, local authority, BDUK and EU development funds.
With micro-details sorted in the budget, the overage coverage of the county gets assessed. Do they meet the 90% target? Can everyone get 2Mbps? Is it feasible? Does it qualify to pass the EU state aid rules?
This turns into a detailed plan, binding both bidder & local authority, where the overall plans determine the eventual adherance to the targets.
To me, it suggests that all the planning of exactly which area gets which solution is already done. A Demand-led rollout now will only affect the order, not the choice.
And given that, for any particular area, it looks to be an 18 month rollout period (after all the interminable planning), is it worth doing? Shouldn't we just leave the successful bidder & authority to decide on the best cost-efficient order?