I hesitated to reply because I don’t want to appear negative. However, there are a couple of things I’d want to get clear if it was me.
In agricultural terms what you are doing would most likely be seen as “ casual labour “. For that the expectation would be that the employer would deduct tax and NI.
IR35 HMRC regulations would expect a self employed person to have other clients for the self employed business ( as a sole trader) . Easily done by taking on a bit of work, say gardening locally.
If it was me, I’d also want to check my own liability insurance, as well as my own accident insurance. As a self- employed person you might be expected to have your own accreditation for some jobs, like spraying and using some machinery.
I hope this is all easily sorted out with the farmer, is it a friend?
I may of course have read this wrong. It may be that you’re doing his book work or similar. In that case I guess that HMRC would understand that you’re building a new business, with one client to start with, but that you decide some aspects of the work, such as the times, or location.
PS I’ve been involved in farming most of my life, and I was self- employed ( in another type of work), for over 20 years.
It’s not unknown of course for farmers ( and other businesses) to “ employ” people “ off the books” . I guess it depends how risk averse you are.