The problem is always going to be money.
Having seen the house of commons fighting over Europe/aid/ and all things financial, defence will be the last thought on their minds.
MPA is important but unless the UK looks at other options they can afford, nothing will happen. The RAF needs to be able to justify 'bang for buck' and with the inter service infighting (to protect pet projects) which always happens when the purse strings are tightened their request must demonstrate cost against capability to stand out from the crowd.
The RAF could have a capable MPA, (maybe not the best) able to perform the role that is required and deal with current or potential threats whilst possessing the ability to expand its capabilities at a future date.
The UK already owns C130J aircraft,Lockeed have produced a good quality roll on roll off MPA package.This option could get the UK back in the air and provide a credible MPA capability at low cost. 4 kits would be cheaper than a P8 and readily available.
Everyone would like P8s, they are an outstanding aircraft but right now in the current political climate, they're too expensive to justify.