It won't happen in the UK. The Food and Environment Protection Act essentially put paid to that.
The work is generally in the US (where you will need to get a work permit), Africa, Aus and NZ (permits again for the last 2)
When I looked into it in Aus/NZ about 10 years ago, the general route in is as a loader driver for a few years; after which you might be allowed to do a few ferries, and eventually some of the easier aplocation work (i.e top dressing from 50')
Bear in mind you will be working for an agricultural contractor who flies planes. Assess how easy it would be for you to get a job as a combine harvester driver if you are not from an agricultural background. If you're from a different background it would be unlikely you'll get a job - the same will apply as an ag pilot - are you there for the agriculture, or the flying?
Try looking at
Agair Update a US based site.