Sea King ZH540 (a Mk 3A) - flown by OC B Flt 22 Sqn, OC SAR Standards and a couple of the SARF's finest door sliders and crowd wavers
Authed to 200ft I believe but flown slightly higher for extra visibility to crowd and TV.
Held at Blackfriars bridge and called in by mobile phone from the palace using Airwave set on the aircraft.
OC B flt's second flight up the Mall, the first was a few years ago taking a SK to land at the palace!