With the investment in Plymouth to support current SSBN I would assume that the safety case would support Sucessor SSBN as well. If they were based in Plymouth then Culdrose, St Mawgan, St Athans etc. become possibilities. That would involve significant investment in infrastructure, but that seems inevitable wherever this MMA capability is required.
Close in defence of the Clyde for SSBN could probably be done effectively by increasing the fleet of RN Merlin Mk2 and basing them at Prestwick, but then that would perhaps undercut the business case for the MMA...
Edit: Concur that I'd put my money on P-8 and Waddo if I was a betting man. Having been round a P-8 a few times it's a good piece of kit and the Seedcorn guys have been instrumental in getting it up and working well.