> Are there cases where a one-person Ltd should do something like this?

Probably. You could retain ownership and rent it to your company. Sell it in return for debt, etc.

I’m guessing these are favourable (potentially) in terms of tax and asset protection in bankruptcy, but also probably have fairness rules that the HMRC would enforce so you’d need to be careful. For example maybe you sell your SaaS to the business for £20M then forever the company is repaying debt to you, instead of paying dividends. There are many issues with this but I’m sure HMRC would pile on their own ones. In general, at the scale you quote, I’d imagine you want simplicity over clever, as the tax differences would likely be small (and possibly eaten up by accountant fees to facilitate more complex transaction).

Re IR35 yes sounds like you’re ok. It’s just a constant shadow looming over you. Anyway sounds like you’re well aware of it.

Clearly, IANAA.