Contractor Among Contractors
If you are a sole trader, there is always a risk of being deemed to be an employee of the engager, with all the problems that could trigger. It's unlikely to happen, but it could. If they are risk averse, you can't do sole trader without lying to them about the risks.
The only way to certainly protect the engager from UK liabilities is to form a limited company. Then, you are an employee or officeholder of the LtdCo, rather than their employee, and so under no circumstances would they be dragged into UK liabilities. The full responsibility (IR35 and everything else) would be on your company.
If you form a limited company, they are protected completely, but IR35 concerns come into play. IR35 is intended to ensure that you pay taxes like an employee if you are one. The reason for this is to prevent you taking advantage of the favourable tax treatment of dividends, as compared to the tax treatment of salary.
In your case, the difference between the two is not a lot, since you are of state pension age and don't have to pay Class 1 NI. For the moment leaving pension contributions and other allowable expenses out of the picture, the difference between operating outside of IR35 and inside is about £2500, on the first £43K of income. That's not nothing, but there may also be ways to lessen it.
In both cases, your first £11K would be paid in salary. You would pay no tax on the first £8K and employer NI (13.8%) on the next £3K. Outside IR35, you would pay dividends above that, and you would pay 20% on the next £5K (Corporation tax), and net 26% (CT plus Div Tax) on the next £27K. If you decided not to fight the IR35 battle, you could instead just pay salary on the next £32K, and it would cost you 33.8% (Employer NI, Income Tax).
For income above £43K, outside IR35 you would pay effectively 46%. Inside, you would pay 53.8%.
Another benefit to a limited company -- you can make pension contributions from the company. This is completely tax-free until you draw down the pension. Pension contributions are exempted from any IR35 considerations. And when you do take it, 25% is tax free, and none of it incurs NI.
If you operate inside IR35, and depending on what your pension situation is, and given your age, you may even be able to get creative in making annual £10K pension contributions from your company and taking an annual £10K pension drawdown. That would move £10K from your company into your hands without any Corporation Tax, any NI liability, and Income Tax only on £7.5K. But you should only do this if you know exactly the ramifications. If you do this even once, you'll never be able to make a contribution larger than £10K again. But that may not be an issue for you.
I am not an accountant or a pensions expert, and this kind of stuff is definitely something for which you should get professional advice.
The key points here, to me, are 1) if you want to strictly protect the NGO, a limited company is the only way to be sure of doing so, and 2) that puts you at risk of IR35, but you can operate inside it and still be pretty tax efficient.
Further to IR35, you might be able to make a case that you are outside. It appears Supervision / Direction / Control is very limited. Managing interns isn't necessarily problematic, I spend a lot of time telling my clients' people what to do. It depends on what that "management" looks like. Telling them how to do a task is fine. Performance reviews, hiring/firing, determining salaries, that's problematic. If they are unpaid/volunteer interns, or very short-term, it's probably a non-issue.
The budgetary control is the kind of thing that would point towards employment. The fact that you've been an employee would point towards employment. The fact that this was your idea rather than theirs is a plus for you. The fact that you are moving off-site and out of the country is a plus for you, especially since at least one other person that did that is not an employee any longer.
I think you could probably beat IR35, but I don't think it is worth it to even try. Since you are state pension age, I'd just form a LtdCo and operate within IR35. Be as tax efficient with pensions as you can. Set your fees high enough to cover your travel expenses -- foreign travel expenses will be allowable under IR35. It's probably better if you pay travel expenses, rather than having them do so, because of the way the deemed calculation payment works -- you get a 5% deduction before allowable expenses are taken off. Structure it as efficiently as you can within IR35 and you'll do fine with it.
One other thing -- if you go LtdCo, perhaps you could keep your eyes open for pieces that you could break off as a separate project and do at a fixed cost. That kind of thing wouldn't be under IR35. So you could operate your main contract under IR35 (and make your pension contributions and expenses out of those funds) but have smaller fixed-price contracts that aren't under IR35.
Ok, I've gone on long enough.