I have just returned from overseas and the US news held several storylines of concern over MPA capability even with a country with an enviable MPA fleet. It appears that Russian submarines have taken to tracking underwater communications cables, paying particular attention to locations upon which repair of a damaged cable would be very difficult, and thus render international communications 'at risk'. Now the cynic in me does wonder why such hostile attention is not being given around the UK where we rely on Cold War underwater listening facilities. Though in all likliehood we ARE also under such a threat. I hasten to add that this is all in the US public domain being overtly voiced by the military hierarchy in protest against austerity in the US Navy.
The timeline for a UK MPA always was decision in 2015 for a resource online by 2020. There is still time for that to happen. FWIW; if we are to leave Europe, we could do a lot worse than restore the pre-WWII military exchange with Japan. The Japanese Navy in those days was steered by UK advisors. Having seen the Japanese MPA at RIAT: I quite liked it, even if it is physically smaller than Poseidon. Four engines = insurance when required / Two engines = worried about the effects of salt ingestion with limited options should an engine fail at very low level.