This sounds complex; not unnaturally if you and your husband are divorcing, you want to look after your own interests.
When you were together, and prior to the 'partnership', if what you did was help your husband on the farm, and the profits of the business were going back into the business via your husband's self-employed status, I don't see how you can now claim your husband was employing you.
During the partnership, it is unclear what changed; partners are normally both self-employed and take earnings out of the business; if you weren't be paid, presumably you had no reason to submit SE tax forms and neither has any NI contributions been paid for you. The way you describe it as 'my husband's farm' suggests the partnership did not exist to you having a stake in the land of the farm?
You are going to have to use a solicitor for your divorce settlement and use him to negotiate for a financial settlement that takes account of your contribution to the joint business - that seems preferable to maintaining that you were retrospectively 'employed'. I don't think you were.