From an interested outsider's point of view, it strikes me that the replacement of Nimrod R by Rivet Joint was much more than a simple change of platform, and that comparisons with P-8 are perhaps not very useful.
Quite apart from the different balance between Elint and Comint on the two aircraft (and the different way in which Elint was undertaken) it strikes me that big chunks of UK infrastructure were closed down in favour of a much closer operational integration with the US.
We seem to have gone from operating a fully independent (but linked in) Sigint system to operating three aircraft that are essentially effectively UK-manned elements of the 55th Wing - seemingly a rather easier proposition, and surely rather quicker to change over to?
I know that at times you might just as well have painted the stars and stripes on the side of the Nimrod R, since it was integrated into their flying programme, used their Int data bases, etc., and could even be tasked by US commands. But 51 could do independent ops (Falklands, anti-IRA, etc.) and I wonder whether it could still, and whether the UK could ever use RJ completely autonomously without the planning, the targets, the "take" and the results going straight (and first) to the US, whereas previously I guess it used to go to the UK (Cheltenham? etc.) first? There are a whole bunch of unanswerable questions - Where is the inflight data downloaded to now? Do the US now even have a veto on when and where and how the UK deploys the aircraft?
Do the UK need an independent, sovereign MPA/ASW capability, or do we want to invest and integrate into the US MPA force? Either could be done with P-8, but other platforms could fulfil the first aim, including the Japanese P1, which promises to be cheaper than P-8 but equally (or perhaps more) capable.
And if we do want the independent, sovereign MPA/ASW capability, then can we afford sufficient P-8s to actually do that? Airbus claim that you can buy twice as many C295 MPAs for half the cost - or four times as many for the same cost. So four P-8As, or 16 C295 MPAs? Eight P-8As or a staggering 32 C295s.