T12 performance
The higher airfield elevation certainly doesn't prevent the T12 from operating at Cottesmore. If you are referring purely to hover and vertical landing performance, then the approx 7 millibar difference between the stations will have a marginal detrimental effect at Cott.
In any case Wittering is easily the better station for Harrier VSTOL trg in my opinion becaue of the multitude of strips and pads, the ramp and the Vigo wood for bouncing around on.
Regards