How low can helicopters fly over London?

There is no single number. Over cities and open-air gatherings, aircraft must stay high enough to land without undue hazard to people and property if an emergency arises. The numeric limits sit in SERA.5005 and the UK's own rules.

The rules in order

Minimum height over congested areas (SERA.3105 as published by the CAA)

Aircraft may not fly over congested areas of cities, towns or open-air gatherings unless at a height that allows an emergency landing without undue hazard to persons or property on the surface, except for take-off/landing or with permission. Numeric 1,000 ft above the highest obstacle within 600 m is in SERA.5005(f)(2), per CAA CAP 3066.
Reference: SERA.3105 (quote: 'unless at such a height as will permit, in the event of an emergency arising, a landing to be made without undue hazard'); SERA.5005(f)(2) per CAP 3066 para 2.8 regulatorylibrary.caa.co.uk

UK exception: the 500 ft rule

In the UK the CAA permits SVFR flights to go below SERA minimum heights provided they stay no closer than 500 ft to any person, vessel, vehicle or structure (ORS4 No.1496 para 7). The AIP repeats that helicopters on the routes may fly up to 1,000 ft above the highest obstacle within 600 m but no closer than 500 ft.
Reference: ORS4 No.1496 para 7(a)(b); SERA.5005(f)(2); CAP 3066 para 2.8 caa.co.uk

Recommended minimum altitudes on routes (2025 change)

The London Helicopter Routes Working Group updated the AIP to give recommended minimum altitudes, for example 1,000 ft amsl for the whole of H4 between Barnes and the Isle of Dogs. CAA targeted AIRAC 07/2025 (10 July 2025) per its timeline.
Reference: CAP 3066 para 1.3 (Dec 2024); AIRAC 07/2025 stated as 'expected' caa.co.uk

Along the Thames

Flying along the Thames within R160

Pilots should generally fly over the river between the high water marks without flying so close to the banks as to cause noise nuisance. Single-engined helicopters that deviate from the river must be able to glide back to it. Pilots should avoid overflying hospitals, palaces, schools and prisons where possible.
Reference: UK AIP EGLL AD 2.22 para 10 aurora.nats.co.uk

Single-engine helicopters at night on H7, H9, H10

H7, H9 (Hayes to Gutteridge) and H10 (Gutteridge to Kew Bridge) are not available to single-engined helicopters at night.
Reference: UK AIP EGLL AD 2.22 para 10 aurora.nats.co.uk

Night restriction on H9 for noise

H9 south of Heathrow is normally unavailable to SVFR helicopters from 2100 to 0800 (2000 to 0700 local summer, as printed) when Heathrow runways 09L/09R are in use, for environmental reasons.
Reference: UK AIP EGLL AD 2.22 route table, H9 note 4 aurora.nats.co.uk