HS4Air, a proposed high-speed rail link connecting HS1 and HS2 via Heathrow and Gatwick airports, was originally estimated at approximately £10 billion when unveiled in 2018 [1]. The proposal was rejected by the UK Department for Transport in December 2018 and never progressed beyond a conceptual stage.

Converting the 2018 figure to 2025 prices requires choosing an appropriate inflation measure. A straightforward consumer price index (CPI) adjustment yields roughly £13.2 billion, representing approximately 32% inflation over seven years [2]. However, construction-sector inflation typically exceeds general CPI because materials, labour, and plant costs have risen faster than consumer prices in recent years. Using construction-specific indices such as BCIS tender-price series or rail/civil-engineering indices—the more appropriate benchmark for major infrastructure projects [3]—the 2018 estimate would likely convert to a range of £13.5–£15.5 billion in 2025 prices.

A realistic 2025 estimate should also account for project-specific factors including design changes, complex tunneling requirements around Heathrow, and supply chain pressures. High-speed rail projects in the UK have historically seen costs escalate significantly beyond simple inflation adjustments when these factors are incorporated. If HS4Air were subject to the same cost-per-mile escalations observed in recent HS2 reviews, the 2025 price could potentially exceed £20 billion.

However, no definitive 2025 price estimate can be produced without detailed geotechnical surveys, environmental impact assessments, and a transparent original budget breakdown. Any figure remains a speculative extrapolation based on other projects rather than project-specific data. For budgeting purposes, the construction-adjusted range of £13.5–£15.5 billion represents the most defensible estimate, though actual costs would depend on the specific BCIS index selected and any contingency uplift applied.