Shytorque;
The one at LHR is triangular. Now how confusing is that?
I hope you will read the AIP before parking there, as the triangle you are referring to is a helicopter aiming point located on a taxiway area
EGLL AD 2.20 Para 5 
5. Helicopter Operations
.
.
ii. The helicopter aiming point is located on the taxiway area east of Link 43. It is marked with an 18 m sided triangle with a conventional 'H'. This aiming point is lit and available for use throughout operational hours. The take-off and climb surface has been protected to 8% to the east and west of the aiming point (see Chart AD 2-EGLL-4-1).
iii. Caution must be exercised when using this aiming point which is on a live taxiway.
iv. Helicopters alighting at the aiming point will ground or air-taxi to the parking areas as directed by ATC.
v. In circumstances as detailed in AD 2.22, Flight procedures, paragraph 9, visual separation will be applied whilst the helicopter is in transit between the holding point and the helicopter aiming point. In all other circumstances, standard separation will be applied.
