Part I and Part II identify two structural problems: no documented tax arrangement at the point of contribution, and no regulatory mechanism to enforce one. This post sketches what such a mechanism might look like, and why the obvious model already exists elsewhere.

The disclosure gap

When you buy a bond, an equity, or a structured product, the thing you are buying has an identifier. An ISIN. A CFI code. A prospectus registered somewhere. The terms are fixed at issuance and do not change without triggering a formal process. If the issuer wants to change the terms, they issue a new instrument.

When you make a SIPP contribution, the tax arrangement you are entering has none of this. No identifier. No versioned terms document. No registry. What you are entering is governed entirely by the statute in force from time to time — and that can change.

The gap is not about taxation per se. HMRC is entitled to offer any tax arrangements it chooses, change what it offers going forward. The gap is about the absence of a mechanism that records which arrangement a taxpayer entered, under what terms, and at what point. Without that, “we are just changing tax law” and “we are rewriting the terms of an existing deal” are indistinguishable — which is exactly the ambiguity that enables the 2027 IHT change.

A short history of identifiers

The financial industry kept running into the same problem: opacity about what instrument existed, who held it, and under what terms. Each crisis revealed the cost of that opacity. Each time, the solution was an identifier standard.

CUSIP (1968) — Committee on Uniform Security Identification Procedures. Created after the US paperwork crisis of the 1960s when brokers could not process trades fast enough because there was no standard way to identify securities across firms. Manual reconciliation was failing at scale.

ISIN (1981, ISO 6166) — International Securities Identification Number. Extended the CUSIP model globally. A 12-character alphanumeric code identifying any security in any market. Managed by national numbering agencies under ANNA (Association of National Numbering Agencies), the international body that coordinates the system.

CFI codes (1997, ISO 10962) — Classification of Financial Instruments. A 6-character code that classifies the type of instrument — equity, debt, entitlement, derivative — and its key structural attributes. Works alongside ISINs: the ISIN says which instrument, the CFI says what kind and what its terms look like.

LEI (2012) — Legal Entity Identifier. Created after the 2008 financial crisis because regulators could not map counterparty exposure in OTC derivatives — nobody knew who owed what to whom because there was no standard identifier for legal entities. The FSB mandated LEIs globally. Now required for any entity that trades financial instruments.

UTI/USI — Unique Trade Identifiers / Unique Swap Identifiers. Post-Dodd-Frank (2010) and EMIR (2012), every derivatives trade receives a unique identifier so regulators can track individual contracts, their terms, and their lifecycle. A UTI is the ISIN for a specific transaction rather than an instrument class.

DTI (2021, ISO 24165) — Digital Token Identifier. Managed by the DTIF, a non-profit operated by Etrading Software, as the registration authority for digital assets and tokens. Notably, this identifier infrastructure was built before regulation of digital assets had fully settled — the same pattern applied proactively rather than in response to crisis.

The pattern is consistent: absence of identifiers → opacity about who holds what under what terms → crisis reveals the cost → identifier standard mandated. Or increasingly, the standard arrives before the crisis.

What a tax arrangement registry would look like

HMRC offers tax arrangements — SIPP, ISA, EIS, SEIS, VCT, and others. Each is a product with terms: what goes in, how it is taxed, what happens on exit or death. Those terms change over time.

A registry would assign each distinct version of each arrangement a unique identifier — call it a Tax Arrangement Identifier (TAI). When a taxpayer enters the arrangement, the contribution is recorded against the TAI in force at that point. If HMRC changes the terms, it issues a new TAI. Existing contributions remain under their original TAI unless the taxpayer affirmatively elects to switch.

This is not novel infrastructure. It is what ANNA, the LEI system, and trade repositories already do for financial instruments. The data model is understood. The governance model is understood. The main missing piece is the policy decision to treat tax arrangements as the bilateral deals they functionally are.

HMRC already assigns identifiers to tax arrangements under DOTAS (Disclosure of Tax Avoidance Schemes) — each notifiable avoidance scheme receives a Scheme Reference Number, reactively, for arrangements HMRC suspects of being aggressive. The infrastructure and precedent exist; they are simply applied selectively to arrangements HMRC dislikes rather than to the mainstream wrappers it actively promotes. HMRC also issues Pension Scheme Tax References (PSTRs) for registered schemes and reference numbers for ISA managers — but these identify the provider, not the version of the tax terms a specific contribution was made under. That is precisely the gap.

Modern HMRC systems — Making Tax Digital, Real Time Information, the pensions dashboards programme — already handle far more complex per-taxpayer data linkages. Versioning tax terms at point of contribution would be a modest extension of existing infrastructure, not a new build.

Under this model, the 2027 IHT change would either have required HMRC to grandfather existing TAIs — leaving pre-change contributions under the old terms — or to have disclosed at the point of contribution that the IHT arrangement was a revocable feature of the current TAI, not a fixed term. Either outcome maps cleanly onto the two scenarios in Part I. What it eliminates is the current ambiguity that allows neither accountability nor redress.

A TAI registry of this kind could reasonably be proposed as part of any future ISA or pension simplification review. The infrastructure exists. The precedent exists. The question is whether transparency about the terms of government-promoted savings deals is treated as a feature or a liability.