Planning ahead

SSDI is taxable in the US if you have enough income.

So if we say that it is not covered by the SS pention Article because it is not SS or a pension, but a benefit payment I think you'll have to deal with it as "Other Income". So it will be taxable in the UK and the US. You links seem to allow it to be treated as non-taxable in the UK and you would just pay tax on it in the US.

That's interesting. So if you do not consider it as income covered by the treaty (in theory), then you're saying US tax would apply.
In the US, SSDI can be nontaxable, or 50% can be taxable, or 85%, depending on your income and how you file. All of which are lower than the 90% taxable in the UK. And if your income is relatively low, it might be taxed at a lower marginal rate in the US than in the UK.
So if it could be legitimately argued, it could result in less tax liability. But it sounds like you think it might be a stretch to apply the rules in this way.

That's interesting. So if you do not consider it as income covered by the treaty (in theory), then you're saying US tax would apply.
In the US, SSDI can be nontaxable, or 50% can be taxable, or 85%, depending on your income and how you file. All of which are lower than the 90% taxable in the UK. And if your income is relatively low, it might be taxed at a lower marginal rate in the US than in the UK.
So if it could be legitimately argued, it could result in less tax liability. But it sounds like you think it might be a stretch to apply the rules in this way.

If it's not covered under Article 17 (pensions and SS) the it is covered by the treaty under "Other Income" Article. Of course you don't necessarily need to apply the treaty and even if you did and the SSDI comes under Other Income the Saving Clause means that the US will tax your SSDI just as it usually would. The only circumstance in which you could avoid that is if SSDI comes under an Article that is exempt from the Savings Clause....that's how regular SS avoids being taxed in the US (Article 17.3).

So let's say the SSDI is "Other Income", you'll be taxed by the US as you would regularly and now you need to make the argument to HMRC that there is no UK tax liability because SSDI is substantially similar in character to a tax free UK benefit. I don't think this is a stretch at all.

Thanks very much for your detailed reply. I think this approach might be worthwhile in my case, but I imagine for some other people, it would not make sense. I'll have to calculate the liability in both cases and see if it's worth the extra complexity.