http://www.official-documents.gov.uk...84/0684_ii.pdf
Seems to suggest that it is resources for through-life support that has been approved until March '25.
On 23 June 2011 the Chief of Defence Materiel signed on behalf of the Secretary of State for Defence a Memorandum of Understanding for Sustainment and Follow-on Development that had been signed by the USA Under-Secretary of Defense (Acquisition, Technology and Logistics) earlier that month, covering a Cooperative Agreement to provide support and capability updates for the USA and UK fleets of Rivet Joint aircraft and ground systems. This agreement runs to 31 March 2025.
One hopes that capability beyond '25 will be subject to ongoing assessment.