The speculation of agricultural droughts is fantasy, but it does seem to come often (it's a fun fear for everyone "what if we starve!!!!!!!!!"). What's more likely is yield per acre will continue to increase. Corn yield per acre at the turn of the century was about 50. 120 years later, it's 4 times as high, but on irrigated land, even higher. I think in England, you may not have seen pivots, but they're common here. How far can they be extended? I don't know, but if there was value in it, the water could be pumped quite a distance from waterways.
If the value of grain crops go up because we're using them more directly, there will be more irrigation to go along with it. The market for corn and the availability of cheap energy has converted a lot of US marginal acres to productive acres. It seems dopey to me in the US as we heavily subsidize corn production and then require it to be used as a blend in fuel (it's a political buyoff) - and that's coming from someone whose family owned a farm until about 14 months ago. It was good for us, I guess - not great for everyone (we were landowners, so the good to us part came in absurd rent value for land).