Soozy, I'm sorry if I hijacked your thread by asking about VAT - it's just where my head is at the moment, that's all. I'll have a bit more of a think and will maybe start another thread in due course.
As Honeyend says, it's your profit you would be taxed on, so provided you can prove that you had costs of at least £500 that you incurred in making the hay (e.g. for fencing, tractor etc), no tax is due.
The question about whether you need to register as self employed really hinges on your intention. To illustrate, let's say we each take a pitch a car boot sale. You bring along a carful of stuff you had lying around in your garage, like old tools, books you've already read, clothes your kids have grown out of, etc etc. I bring along a car full of stuff I've bought elsewhere cheaply in order to sell at a profit, plus some jars of jam I've made specifically to sell.
The upshot of that one is that your income from the car boot is 'incidental', and doesn't have to be declared, whereas my income has been incurred in the course of running a business activity (i.e. it was not incidental), thus I need to declare it, and pay tax on my profit (i.e. selling price minus buying price and costs). Exactly the same rules apply for selling on Ebay BTW.
To put that into the context of a smallholding, selling surplus eggs and courgettes at your gate would be considered incidental income. However, keeping extra hens or a polytunnel in order to produce food for sale at hopefully a profit would be considered a business.
Does that help?