Hi, had a quick google around and found this on Parliament's website
House of Commons Hansard Written Answers for 01 July 2010 (pt 0007)
Angus Robertson: To ask the Secretary of State for Defence if he will estimate the cost of cancelling the (a) purchase of the RC-135 Rivet Joint aircraft and (b) Future Rapid Effect System Specialist Vehicles contract. [4597] Peter Luff: The overall cost of the RC-135 Rivet Joint Project, through life, is circa £1.2 billion and at this early stage of the project life cycle any cancellation costs would be a small fraction of this. Rivet Joint is being acquired through the US Foreign Military Sales process which allows customers to withdraw at any time on payment of all costs incurred to date, and any unavoidable liabilities held by the US Government and its contractors.
On 1 July 2010, the Ministry of Defence announced that a demonstration phase contract for Specialist Vehicles had been signed. The cost of a subsequent cancellation would be agreed with the contractor with the Department paying fair and reasonable costs and liabilities at the time of the cancellation.
Whether this is the source of the rumours I don't know.