Based on my understanding of EU law rather than specific knowledge of the BDUK process, I suspect that the companies are asked for a declaration of their intentions for commercial roll-out on a specific date, as state aid cannot be used to compete with commercial enterprise.
A company's declaration cannot represent a contractually binding commitment to develop such an area, as there's no consideration (price or promise) flowing to the company in return for their promise. Consideration is a fundamental requirement of a contract.
The doctrine of promissory estoppel is equally inapplicable to a promise to develop superfast broadband. Promissory estoppel arises when detrimental reliance is made on a promise, but is limited to situations where the relying party places reliance in a promise by the other party to give up their legal rights.
For example, if I agreed to cancel a debt you owed me free of charge to you (so there was no consideration) and we did not make this agreement as a deed (which would overcome the lack of consideration), there would be no contract to cancel the debt that you could rely on. However, if you subsequently made financial commitments relying on my cancellation of the debt, a court would likely hold that I was estopped by my promise if I later demanded repayment.
The learning point is that if there is any hint an agreement might not be backed by consideration and might not therefore be a contract, you should either insist the promise is made in return for consideration - which need only be a penny or a peppercorn (consideration must be adequate but need not be sufficient) - or ensure it is made as a deed (which overcomes the requirement for consideration).
A promise by a company to develop superfast broadband in an area is a statement of intent based on the best knowledge available at the time, rather than the company giving up any sort of legal rights it has over the council or anyone else. In any event, estoppel is a matter of equity (roughly speaking, what is most just) rather than a right, and I'm sure any court would accept it as fair, just and reasonable for the company to change its plans for sound commercial reasons.
Accordingly, I expect declarations of rollout plans by the companies are used solely to exclude areas for consideration of state aid during the planning process. If companies later change their plans, there's no legal reason I can think of why the BDUK project cannot revisit its decisions, though there comes a point where the planning is too advanced for changes to be made. Eventually the available money is committed as seems best and contracts are signed.
With any process as complex as the commercial superfast broadband rollout and the BDUK project, there will always be some examples of hopes being raised and dashed. I don't share in Bob's conspiracy theories - I believe BT Openreach are doing their best to roll out a product that many want to buy via the retail providers. Openreach can only expend resources on rolling out where there is reasonable expectation of a commercial return on their investment (otherwise they do not have a viable proposition for potential investors in the project), or where justified by external income such as BDUK and contributions from developers.