Mate - you've opened a can of worms. The subject is still very emotive even though our Harriers are now in bits in the USA. It comes down to the simple fact that to save money an entire fleet had to go. The one that remained had to complement Typhoons capabilities and make up for slow progress in turning that platform into a mature multi role type.
Here's what the Parliament paper says:
Reduction in the number of fast jet fleets
1.8 Affordability constraints meant that as part of the Strategic Defence and Security Review, one of either the existing Tornado or Harrier fast jet fleets would have to be retired. The choice was discussed in both National Security Council meetings and the National Security Secretariat briefings clearly set out the implications of either choice. In terms of overall contribution to United Kingdom fast jet capability and operations in Afghanistan, Tornado was assessed as more capable. Harrier would be the preferred choice if a continuous carrier strike capability was maintained and would better support the immediate establishment of a UK-French Maritime Task-force. Retiring Tornado would save £380 million less than Harrier over the four year Comprehensive Spending Review period but £620 million more over 10 years.
The upgrade from STOVL carrier to CV carrier was mitigated by the CVS and Harrier savings, but crucially as well by £1 billion by cancelling DPOC - the GR4 replacement.
Please let's not get into the debate again, the Harrier's gone (literally).