Unfortunately it always seems the Treasury puts the boot in and can only be persuaded to fund an 'upgrade' - ie Shackleton Mk 2 to Mk 3, Nimrod 2 to 4, Sea King (too various to mention)and the same with many other items across the military spectrum. So we end up with cost overruns, old equipment with (relatively) modern upgrades etc, instead of new build, more efficient and in the long run cheaper aircraft etc. And who gets the blame - MoD or the single service involved.
There is an old adage.
Never modify a mod. Not set in stone, but there to warn of inherent risks that must be mitigated before being allowed to commence development. In the mid-90s, you had to disobey direct orders to let such a risk reduction contract. That is why SKAEW was such a relative success, as it faced the same risks as Nimrod yet delivered on time, under cost and to a better spec than the FAA asked for. (And was the lead programme on some new technologies common to both). What MoD will not tolerate is using such successes as a benchmark, as it raises the bar.