Savings income grows far less from 2023-24 to 2025-26 than the OBR's savings income forecast #2722

The model's savings income reaches 2025-26 on a much lower path than the OBR's savings income forecast.

• The SPI-calibrated base is 2023-24. microcosm carries it to the 2024-25 dataset year by ons.household_interest_income (ONS D.41, ×1.142 on the 2.162.8 index), and dataset projection then grows savings_interest_income, other_investment_income and non_cash_isa_cash_interest by obr.per_capita.gdp (×1.047 into 2025-26, plus population for the weights).
• The OBR's "Savings income" growth (October 2026 EFO, receipts Table 4.5, pre-measures) is 34.96% in 2024-25 and 0.65% in 2025-26: ×1.358 over the two years.

The low 2025-26 level feeds every later year, since from 2026-27 both paths grow by GDP per head. It lowers the yield of any savings tax change, such as the 2-point rise in the savings rates from April 2027.

Which series should carry 2023-24 savings income to 2025-26 is open: the OBR's series is the savings income in its self-assessment receipts forecast, while deposit-based measures (Bank of England effective rates times balances, less cash ISAs) rise much less. #2638 proposes D.41 interest received per person instead.