The UN insisted on contracting me personally, not through my company.
I have found plenty of information about the tax position and conditions for exemption related to employment overseas, but nothing at all relevant to consulting work done in the UK for overseas organisations such as the UN, NATO, or EU.
I was told (by a UN official in Geneva) that the consulting fee was tax-exempt. However, unable to find any information on the HMRC website. Maybe they just meant that it was exempt from Swiss tax?