PA,
P-8 - In service, yes with limitations and falls short, in significant areas, of satisying UK requirements.
proven - yes but did not get a glowing report during OT&E.
tested - see above.
biggest - fuel hungry. Requires complex infrastructure.
best - less than 80 % of requirement.
already operated by RAF aircrew. Irrelevant. Will provide some mitigation in terms of training but nothing else.
Can replace Sentinel and E-3 (Wedgetail). Requirement for Sentinel is still, being debated. WedgeTail sub optimal.
I would say P8 satisfies less than 70% of requirement.
To get it to 80% will be a massive extras cost that will require further testing etc making an expensive platform even more expensive.