It is not disguised interest under s381A because the amount of indexation bears no relationship to the time value of money or a commercial rate of interest: subs (4)(a) and (b).
I regret to say that although I accept entirely and have read all the sources you cite I believe HMRC are wrong. Many clients are apt to accept the official view just because it is such. Entirely understandable. For me, and many of my former clients, if we thought they were wrong and the cost/benefit was favourable we would be prepared to take them on. Some had to be held back!
One example. A UK resident subsidiary issued a deep discount security to its parent company, resident in a non-treaty company, because it avoided withholding tax. It was issued on pinpoint commercial terms backed by banking opinions so as not to offend transfer pricing. Instead of issuing one 5 year security five were issued for 1, 2 , 3, 4 and 5 years respectively so that the CT deduction would fall into each future AP of repayment and not only just in AP5. HMRC did not like it but had to accept it. If HMRC had previously announced (legislation by proclamation) that this did not work my particular clients would still have gone ahead because no one on the Board wore brown trousers, the reward was really significant, and the alternative of a straight loan at interest very unattractive.
HMRC’s position on anything is a formidable fact to be recognised and a client must be warned about it, and indeed that HMRC may disown it if it suits them. To do something that will challenge it needs careful evaluation of cost, hassle, uncertainty of and delay in outcome, inequality of arms as to funding endless appeals even if TWM, unwanted publicity, blacklisting after victory, and trustee clients are not staking their own money. But an adviser’s duty is to point out all these things and the likelihood that HMRC are wrong in law. An adviser that does not do so is in a spot if someone else takes them on and wins; even if that is not actionable it will not be great for his or her ongoing client relationship and/or perhaps professional reputation.
Furthermore it is most unlikely that trustee loans of this kind would be caught by the GAAR and that they are made without reference to tax avoidance as a legitimate choice of route; to maintain the value of the money lent which would not be compensated for merely by a commercial rate of interest. This is economic reality, the very lifeblood of the capitalist marketplace, Reverse Trussonomy. Why is it do you think that HMRC have not bothered to obtain legislation to enact their position since 2017 but have relied on an ex cathedra encycyical? My guess is that they are not confident of it, so in terrorem is a better strategy and that this has been swallowed whole by trustees and their advisers which is why there is no case law. So the threat has worked! File problem in out tray.
No client, especially not trustees, wants to blaze a trail at the cost of the trust fund and largely for the gratuitous benefit of others but I am surprised that apparently no trustees, finding themselves in the situation, have pressed an appeal at least to the FTT where they will not have to pay HMRC’s costs and, as many taxpayers do, have represented themselves or forgone representation, and relied on the probity and self-evidently trustworthy legal knowledge of the Tribunal judges. HMRC have an unfair advantage in being not only one party to every tax case but able to investigate all potential cases and manipulate the list to get on first a weak case for the taxpayer.(Oh no they don’t; Oh yes they do). Even so will someone please give it a go? And also on many of the other different issues that HMRC decide unconstitutionally by official dictat.
Jack Harper