The 737AEW&C or Wedgetail may be the best choice.
I do not believe that the UK has taken the necessary steps to determine that this is the case, however.
The UAE decided that the 737AEW&C was an inferior solution to the Globaleye (Global Express with Erieye ER). They were offered Erieye, 737AEW&C and E-2D and selected Globaleye. Many sources suggest that the 737AEW&C didn't even come second! That should surely indicate that some proper analysis/evaluation of Globaleye would have been appropriate, before making a decision in favour of an uncompeted, sole source procurement of Wedgetail?
Many air forces are turning away from big, airliner-based AEW&C and ISR platforms in favour of biz-jet based solutions. Italy, Singapore, Israel, for starters, with some indication that the US might also follow that approach. That should surely indicate that some proper analysis/evaluation of bizjet platforms would have been appropriate, before making a decision in favour of an uncompeted, sole source procurement of Wedgetail?
And if there are compelling reasons to go down the route of having a bigger, airliner-based solution, then one based on Voyager, with higher UK content than a 737, and with RR engines, should at least have been properly evaluated, before making a decision in favour of an uncompeted, sole source procurement of Wedgetail?
The Turks don't seem to have been very impressed by 737AEW&C so far, and IIRC, nor were the Aussies, early on. The E-7/737AEW&C radar has been out of production for the best part of a decade, while Erieye has undergone constant development and improvement, and has been proven in Europe by the Swedes and a NATO ally, Greece, as well as in Brazil, Pakistan, the UAE and Saudi Arabia. Sorry. 'Country X'. That should surely indicate that some proper analysis/evaluation of the Erieye ER radar would have been appropriate, before making a decision in favour of an uncompeted, sole source procurement of Wedgetail?
I understand that the planned Aussie upgrade of Wedgetail may be transformational, and may lift the 737AEW&C into a clear, class-leading position, but it is far from clear that the 737AEW&C, as it exists TODAY, enjoys anything like the superiority that has been claimed by some and implied by others.
It is also clear that there has been no serious analysis or evaluation of alternatives to Wedgetail. The refusal to accept classified Erieye data is jaw-dropping and shocking.
After the similarly uncompeted order for Boeing P-8A Poseidons, more Boeing Chinooks, and the equally uncompeted order for Boeing AH-64E Apache Guardians, this looks awful, and for the sake of appearances alone, more than a cursory look at options would surely have been appropriate, before making a decision in favour of an uncompeted, sole source procurement of Wedgetail?
Getting one of AOC ISTAR's Wing Commanders to have a quick look at Google and Wikipedia, incidentally, doesn't count as a serious analysis or evaluation of alternatives......