JIV, there is slightly more to the E-3D story though: we had access to international upgrade programmes, which split the cost between as many nations (or operators perhaps as NATO aren't a nation) partake, but haven't really used this option to keep pace with the other E-3 fleets.
We won't commit to the budget to upgrade significantly, but accepting the potential compromises of international mods could have been significantly cheaper than pursuing UK specific mods.
It doesn't seem impossible for P1 or P8 to be run on a similar programme for collaborative mods.