Apparently the Sentry upgrade program will cost in the region of ~£2b, and it will be near to 2030 by the time the entire fleet is upgraded (for a 2035 OSD).
In light of those facts, I think we ought to look at replacing the Sentry fleet entirely.
The 737 AEW&C platform (the E-7 Wedgetail in RAAF service) would be a good candidate as it has a lot of commonality with the P-8 (same engines, cockpit, and many other systems), which would create some good maintenance and aircrew training synergies.
If however we're going to keep Sentinel long-term, then the Saab GlobalEye would be another good (and cheaper) candidate, as it is based on the same airframe as Sentinel (Global Express), which again would give many synergies between the two platforms. Saab seem to think that they could sell some to the RAF:
https://www.flightglobal.com/news/ar...stomer-429604/