Pre-2010 Plans for Harrier
Prior to the general election of 2010, the government had intended to remove Tornado GR4 from service by 2015, ten years earlier than planned and use the savings from this to fund putting upgrades onto Typhoon. They also planned to keep carrier strike until the QE class carrier was ready and then embark the Harriers onto the new QE class carrier until F-35 was ready.
Defence Equipment 2008
“The MoD plans to keep the Harrier GR9 aircraft in service until around 2018 and to operate these aircraft from the new carriers.”
Defence Equipment 2009
“The second question you ask is what does that mean for the existing three carriers. The Invincible is already at some notice in fact; Ark Royal will probably be withdrawn from service before too long, in the course of the next few years, and we will need to have Illustrious certainly remain in-service until it is quite clear that the Queen Elizabeth has passed her sea trials and that her aircraft complement, whether they are still Harriers or JSFs at that stage, are fully worked up and operational."
The OSD for GR4 was 2025 and logistic support of GR4 from April 2011 was put at £4.8 billion, in Nov. 2010.
The savings from removing carrier strike (including Harrier) were put at £1.6 billion; the savings specifically from removing Harrier were put at £0.9 billion to 2018.
The saving from retiring Tornado GR4 early could have paid for carrier strike to be maintained, by extending the OSD of at least one carrier and maintaining Harrier until JSF was available. It also would have left a considerable sum to pay costs of Typhoon air to surface upgrades (Storm Shadow and DMS Brimstone for example; later included with other upgrades in Project Centurion costing about £425m).
The pre-2010 plan to maintain carrier strike and update Typhoon by retiring GR4 in 2015 looks a reasonable option financially.