I think it's pretty simple. Why buy a GR9 and a GR4 replacement, when you can do both at once.
I can see a future with 5 Typhoon and 5 Dave C squadrons being the total UK fast jet fleet.
Once that decision is made, the harriers can go straight away. The government can explain that they have decided to move to the more capable CTOL configuration, therefore VTOL skills no longer need to be retained.
Those 12 Merlin HM1s in store (not part of the upgrade) and probably earmarked for the AEW role, can then be immediately converted for a more utilitarian role..... and the government can claim that they are getting helicopters to the front line.
... and finally the treasury gets a wedge back to plough into the NHS.
