I don't think the procurement will be quick as the Japanese have a towel to throw in as do other countries
Maybe for P8s, but if you're looking for a more immediate fix and can't wait in the P8 queue, then it may be time to consider capability rather than platform and see what that throws up. Just some ideas off the top of my head:
Buy more MQ-9 / some MQ-9 Block 1+ or RQ-4N and park them at Kinloss & St Mawgan for specific MPA duties (can't see the Army being happy about the former, but security needs would have to take priority over inter-service whinging)
Look at alternative manned capabilities as a temporary fix. Aren't Lockheed trying to plug the re-introduction of the S-3 Viking? Didn't the Italians buy some ATR-72s as a stopgap measure?
Upgrade / re-role existing platforms. Convert Sentinel from a bespoke GMTI platform to a MMA platform. But this would be a relatively low tech solution and you would probably still need another platform to do the handle the more advanced and kinetic side of things.
I believe this last option is what will be touted as a solution under SDSR 15 given Philip Dunne's answer to the HoC on 23 Oct when asked about a life extension for Sentinel:
With regards to the signature of contracts for the life extension of the Sentinel aircraft fleet, I refer the hon. Member to the answer I gave on 1 September 2014 (Official Report, column 103W). To date, no related costs have been incurred. On current plans, a contract for the development and installation of the maritime capable software upgrade will be let in spring 2015. Sentinel was already fitted with a Dual Mode Radar when it entered service with the Royal Air Force
Whether this is a temporary fix to allow the Govt to say they are re-introducing an MPA capability I don't know, but this would only work if you also have the capability to do something about the threat other than just watching it sink your subs and carrier.