My partner is the sole director and shareholder of a limited company in the UK. I'm a higher tax payer working for another company and currently receiving just PAYE. Would would be the most tax efficient way to help out my partner? The service I'm offering are just IT related and specifically the creation of a software while my partner will take care of marketing. Can I just do that as a volunteer? Does my work needs to be formalized somehow? What happens if I become a company secretary? Will either me or my partner have a higher tax burden? I know that if I become a director there may be issues because after £2,000 of dividends I'll be heavily taxed.
The simplest way to accomplish your immediate goal of giving software to your partner's company is to assign them the copyright in return for a nominal fee (e.g £1).
However, unromantic as it may be, you should look to your own position here. If you and your partner split up then you will be left without any rights to your work (except anything potentially awarded by a divorce court). You should consider how this might play out in the future.
The best option in my opinion is to buy a share in the company, and get dividends when the company makes money. You will be taxed of course. And there must be a reason for the dividend, that’s why you must be not just company director, but a major shareholder. Dividends are not paid for work done, but profits distributed to shareholders.
But that’s just my opinion, I’d strongly recommend asking a professional.