Pillar 2 is the global minimum tax: multinational groups with consolidated revenue of €750 million or more must pay an effective rate of at least 15% in every jurisdiction they operate in, with any shortfall collected as "top-up tax". The UK has implemented the full set — the Multinational Top-up Tax, a Domestic Top-up Tax, and since the end of 2024 the backstop UTPR — and the regime has now crossed from legislation into live compliance: registrations are done, and the first UK returns fell due in mid-2026.
For groups near the threshold, and for the UK subsidiaries of larger ones, this is no longer a policy story. It is a filing calendar. Here is where things stand.
The UK's three-part implementation
For accounting periods beginning on or after 31 December 2023, the UK operates:
- Multinational Top-up Tax (MTT) — the income inclusion rule: UK parent entities pay top-up tax on profits of foreign subsidiaries taxed below 15%;
- Domestic Top-up Tax (DTT) — the UK taxing its own low-taxed UK profits first, so the top-up stays here rather than being collected abroad. DTT applies to large domestic-only groups too, not just multinationals;
- UTPR — from periods beginning on or after 31 December 2024, the backstop that catches profits escaping the other two rules, typically where the ultimate parent sits in a non-implementing jurisdiction.
The 15% test is not your headline corporation tax rate — it is the GloBE effective rate, computed on adjusted accounting profits per jurisdiction. A UK company paying 25% corporation tax can still produce a sub-15% GloBE rate in a bad year, courtesy of timing differences, super-deductions and credits; "we're in a 25% country" is the most common false comfort in this area.
The compliance calendar that has already started
Three obligations, with dates that have begun landing:
- Registration: in-scope groups register with HMRC within six months of the end of their first in-scope period — December 2024 year-ends were due by 30 June 2025;
- GloBE Information Return (GIR): the detailed data return, due 18 months after the first in-scope period ends (15 months thereafter). For December 2024 year-ends that meant 30 June 2026, with HMRC waiving late-filing penalties for returns in by the end of July 2026 as a transitional soft landing. Where the GIR is filed in another exchanging jurisdiction, the UK accepts a notification instead;
- The UK self-assessment return and payment alongside.
Groups that treated the June 2026 deadline as a crisis discovered the real lesson: the GIR wants data — jurisdiction-by-jurisdiction accounting profits, covered taxes, payroll and asset carve-outs — that finance systems were never designed to produce. The second filing is easier only for those who industrialised the first.
Safe harbours: the workload valve
Most groups' first returns lean heavily on the transitional CbCR safe harbour: where country-by-country data shows a jurisdiction passes a simplified test — a de minimis, a simplified effective rate (17% for fiscal 2026), or a routine-profits test — the full GloBE calculation for that jurisdiction is switched off. The transitional regime covers periods beginning on or before 31 December 2026, so its end is now visible: 2027 was going to mean full calculations everywhere, which is why the draft UK legislation extending relief and adding a permanent simplified-ETR safe harbour — applying retrospectively from January 2026 — matters so much. It is draft, not law; plan on the current rules, watch the extension.
The American question
The unstable piece of the architecture has been the United States. Following the 2025 G7 accord, the OECD Inclusive Framework agreed in January 2026 a "side-by-side" safe harbour: groups whose ultimate parent sits in a qualifying regime — currently only the US — have their IIR and UTPR top-up deemed zero for fiscal years from 2026, in recognition of the US minimum-tax system running alongside. Two caveats keep this from being the all-clear US-parented groups sometimes hear about. Domestic top-up taxes are not switched off — a US-parented group's low-taxed UK profits still face the UK DTT — and the UK legislation enacting the safe harbour is still in draft. Meanwhile Pillar 1, the companion reallocation of taxing rights, remains effectively stalled.
What this means below and above the line
For groups near €750 million — organically or via acquisition — scope monitoring belongs in the deal model and the board pack: the test looks at two of the previous four years, so growth today creates obligations with a lag. For clearly in-scope groups, the work has moved to data plumbing, safe-harbour elections and keeping pace with a rulebook that has changed every year since enactment. And for UK subsidiaries of foreign groups, the local job is real even when the parent leads: the UK registration, the DTT exposure, and supplying group reporting with UK data on the GloBE's definitions rather than the statutory accounts'.
Accounting-wise, current top-up tax is recognised and disclosed in the tax line as it arises — the IAS 12 exception spares groups only the Pillar 2 deferred tax gymnastics, not the current charge or the disclosures, and auditors now ask Pillar 2 questions as standard at year-end — another reason the calculation cannot live in one specialist's spreadsheet.
Acumon supports UK entities of in-scope groups on registration, DTT analysis, safe-harbour testing and the UK end of GIR data — through our international tax team, alongside corporation tax compliance and UK subsidiary services for inbound groups. If your group brushed past €750 million in recent accounts, the scope check is a short piece of work with a long deadline tail — worth doing before the deadlines do it for you.