An important report led by the Governnment's Chief Scientific Adviser, Sir John Beddington, setting out the challenges facing farming and food supply on a global basis is now available: Farming Future You can hear a Radio 4 discussion on the topic here: Beddington
This should provide a basis for a serious discussion about how agricultural productivity can be raised whilst coping with the challenge of climate change and other environmental considerations such as the maintenance of biodiversity.
Land is a finite resource, indeed its availability is diminishing because of urbanisation and the effects of climate change. Farming and food is very dependent on oil at various stages of the food chain, while the availability of water is an increasing constraint.
In the longer run support for the farming industry should not come through blanket subsidies but by, for example, ensuring that there is an adequate research structure that is oriented towards devising practical solutions towards the resolution of pressing problems.