Ok, let's re-do EngO's post for 1991:
Tornado GR1: 2, 9, 13, 14, 15, 16, 17, 20, 27, 31, 617 - total 11
Tornado F3: 5, 11, 23, 25, 29, 43, 111 - total 7
Jaguar: 6, 41, 54 - total 3
Harrier: 1, 3, 4 - total 3
Phantom: 19, 56, 74, 92 - total 4
Buccaneer: 12, 208 - total 2
Total 30 Front Line Combat Ready Fast Jet Sqns 
In addition to the above, there were the following OCUs:
Tornado GR1 TWCU: 45(R)
Tornado GR1 TTTE
Tornado F3: 229 OCU (65 Sqn)
Phantom: 228 OCU (64 Sqn)
Jaguar: 226 OCU
Harrier: 233 OCU
Buccaneer: 237 OCU
Total OCUs 7 Front Line Limited Combat Ready Fast Jet Sqns
and 1435 Flt in the Falklands with 4 x Phantom FGR2 - let's call it 1/2 a Sqn
So in 1991, the UK could raise 37.5 Fast Jet Sqns
How many today in 2014?
4x Typhoon
3x Tornado GR4
1x Typhoon OCU
1x Tornado GR4 OCU
1435 Flt in the Falklands
9.5 Sqns in total in 2014 versus 37.5 Sqns in 1991 - that's ~25% left!!!
So in answer to the OP's question - it's reduced by 75%. However, on 1 Apr 14 we lose 12 Sqn and that will be pretty close to 80%
LJ