James,
You are confusing the following (my definitions):
Total airframes - those still in Sqn use, deep maintenance, long term storage, etc...
Serviceable aircraft - those currently on front line Sqns, OCU, etc that are fit to fly.
FE@R - Force Elements at Readiness. I don't have the exact definition to hand, but it refers to the number planned to be deployable on a long term sustainable basis, i.e. the number deployed that could be sustained for months/years if necessary (obviously with rotations of crews and individual airframes)
When you consider that for any military force (Army, Navy or RAF) they say you need at least 3x the deployable numbers, one set deployed, one set working up, the other set recovering after just getting back, then the FE@R figure is unlikely to be more than a third of your total available assets.
Thus they were saying that for the Harrier fleet the FE@R figure was 10. However, the total number of airframes, either serviceable or unserviceable, would be considerably more.
Hope that clarifies things a bit.....