The House of Commons Library publishes from time to time a Briefing Paper on “Retrospective Taxation” by Antony Sealy, SN04369. The latest edition is 8 April 2020. https://commonslibrary.parliament.uk/research-briefings/sn04369/
This charts HMG’s historic attitude to the subject. Broadly HMG considers that it can do whatever it likes but regards itself as voluntarily restrained within stated parameters (a “self-denying ordinance” but note that in 1645 such a measure did not extend to “good chaps” such as Cromwell). So far the judiciary has not discovered a case in which the latitude accorded to HMG in tyrannising individual taxpayers retrospectively by the HRA 1998 has been juridically exceeded. Theoretically a failure to observe its own self-imposed restraints might possibly be amenable to JR especially after Miller No 2.
The particular issue raised here would certainly appear to be on the side of the angels. There is no nasty contrivance or artificial scheme. There could be no question of restoring the “original” view of the legislation (i.e. that which HMRC holds, at least now if not earlier). There has been no official warning against the specific behaviour nor can it be fairly said that advisers and other sages have created an undeniable impression in taxpayers’ minds that it could well be clobbered. These have all been proffered by HMG as past justifications.
A crystal ball has long been an essential piece of kit for tax advisers (plus a disclaimer!) and they now need peerless metaphysical divination as to what conduct conforms with the the understanding of pristine fiscal purity vouchsafed from the mountain by the PRCT and SRA.
Even so it will be a dilemma advising whether the present lack of warning will likely save someone who embarks now upon this type of (allegedly) hitherto Government-sponsored tax mitigation/planning (surely not “avoidance”, cherubs) and for how long that situation might prevail. Fortune may favour the brave. Those who have already acted may well be secure. Like “deprivation of assets” under the CRAG, acting early may defuse in due course even the robust judgment of hindsight from the great and good.
In the final analysis HMG is only motivated by political expediency. Some sorts of public opinion may prompt or restrain action or induce U turns. Wealthy taxpayers and their advisers do not have a great track record as influencers of fiscal policy even when 150% of GDP is not the current deficit.
The main thing these days is that taxpayers should be totally realistic about the probability and cost of the downside of planning and enured to the philosophy that, in the modern fiscal fashion industry, nothing lasts forever (e.g Entrepreneurs’ Relief). The glass may not be even half empty.