I'm not sure that a PPL pilot could 'legally' fly a maritime surveillance sortie for the RN/RNR due to the restrictions of the PPL. To carry out any form of paid flying work, the pilot must hold either a CPL or an ATPL with appropriate type ratings. The only exception are military pilot's who have a certificate from the MAA.
I suppose that if a PPL pilot was given a certificate by the MAA to authorise for flying duties, then legally speaking the pilot would be flying on his MAA certificate and not the PPL. However, that's going to involve prospective pilot's carrying out the same flying training as regular military pilots! I agree it would be great for the PPL pilot to receive (and get paid) for this training, but I doubt the treasury will share the same enthusiasm.