NFN - there is no way that agricultural land in England will fetch £15,000/acre following a crop failure in Outer Gongolia. Average price is less than £10,000/hectare, which equates to £4050/acre.
That said you won't get amenity land (land that can not be built on, but for which agriculture is not the prime purpose) for that price - expect to pay three times the price and more, depending on how much the seller thinks you want it/can afford.
Strutt and Parker say avg = £3061/acre