We were the successful tenderer but the agency has withdrawn the contract offer

It depends on the terms of the tender documents and what was communicated. Most requests for tender reserve the right to cancel the process or not to award a contract, and these reservations are generally enforceable. An announcement that you were the preferred supplier is not usually a binding contract unless it was an unconditional acceptance. However, if there was a concluded agreement, or if the agency acted in bad faith, made misleading statements or breached its own process, there may be claims in contract, in misrepresentation under the Contract and Commercial Law Act 2017 or under the Fair Trading Act 1986. Review the tender terms and all correspondence and keep records of costs. Seek legal advice quickly. A debrief and a written explanation from the agency should be requested.