As much as we like the idea of incremental acquisition we do not have a process that supports it.
We have Defence Strategic Direction (DSD) which says that we should fight wars on Mars. We know that we only have the technology to fly to the Moon for £xM but that doesn't meet the requirement by a long way as the URD/SRD/OA is based on Mars. By doing studies we accept the risk that the capability gap to Mars is too far and won't do anything until 2025 until Mars aircraft become available.
In contrast we believe good practice is UORs where users say we want that thing and we will operate differently to use that thing. Its available today at a known price but has no link to DSD or OA.
The answer to the Maritime ISTAR question is not what you would ideally like to do but given an off the shelf platform what could you do. Sadly that is not the way it is sold on VFM grounds.