Known as the HSAB, High Speed Air Blast helmet, it was designed for high speed ejections, intended for use in the Tornado, but I believe never entered full service.
Made by ML Aviation Ltd, resembling a motorcycle crash helmet, it was a one-piece helmet providing impact protection, wind blast protection, communications, and oxygen via an in-built mask.
I took this imaget at Newark air Museum's CockpitFest 2009, it belongs to a friend.
Taken from a 1966 Flight International Magazine, ML Aviation advertise the Mk5 HSAB, and the High Altitude ML12P.
I found this image below in an unidentified article by S.J. Lindstone, note the wearer also is wearing a torso harness.