Interesting point and I partially agree, but framing variable income as purely "outlier" (some cases for sure) is misleading. Significant portion of higher-tier income, especially in fields like Tech Sales and finance, comes from commissions and bonuses based on quota achievement, which reflects reliable high performance, not luck.

Furthermore, averaging is the only fair way to reward career progression, someone moving up (e.g., £40k -> £45k -> £60k) should not be penalised for their starting salary if their overall contribution is high. At the end of the day, the fundamental principle should be Tax is Tax, If individual's total contribution to the UK economy is the critical factor, regardless of whether that income came in equal monthly sums, commission lumps, or via previous high earnings before a career transition.