For promissory estoppel to apply, should the promisee have acted to his detriment?

Yes, for the doctrine of promissory estoppel to apply in English law, the promisee must have acted to their detriment in reliance on the promise made by the promisor. The doctrine of promissory estoppel operates as a defense against the enforcement of a contract in circumstances where a party has relied on a promise made by another party, and has suffered a detrimental change of position as a result. This means that the promisee must have taken some action or refrained from taking action in reliance on the promise, and must have suffered some loss or disadvantage as a result. The doctrine of promissory estoppel requires a causal link between the promise and the detrimental change of position suffered by the promisee, and is only available in circumstances where the promisee can show that they relied on the promise to their detriment.