It does seem to be the Royal Navy/Royal Marines who used armed Wessex helicopters rather than the RAF.
Armed Wessex transports
There were two configurations for armed Wessex transports. Some were armed with a single GPMG mounted on a swivel fitted to fire from the starboard side entrance door or window. Others carried two pintle-type mounted GPMGs - one behind the rear window on each side.
A ‘Junglie’ picking up a Special Forces patrol during the confrontation with Indonesia. It is armed with two GPMGs. One can be seen in the starboard rear window:
A later photo taken during the Aden insurgency:
http://www.royalnavy.mod.uk/operatio...imageIndex/15/
AFAIK armed transports were available from the beginning. They seem to have become more common as time went on though - probably because they were often armed or up-gunned before deployment to hotspots (such as Aden, Borneo, Oman and the Falklands).
Attack helicopters
The Royal Navy/Royal Marines armed some of their Commando Wessex Mk 1s and all of there Wessex HU.5s with a wide variety of weapons. They could be used to attack either naval or land targets. These seem to be two of the main loadouts for land operations:
1) Commando support
2x 2.75 inch rocket pods
2x SS-1 ATGMs
2x 7.62 mm fixed forward firing guns
1x rear mounted GPMG (sometimes x2)
A photo:
2) Anti-tank
4x SS-1 ATGMs
2x 7.62 mm fixed forward firing guns
1x rear mounted GPMG (sometimes x2)
Their armament is similar to that of other British helicopters at the time: the Westland Scout, Wasp and Whirlwind.
The first Wessex squadron to be equipped with attack helicopters was commissioned in April 1962 and at sea on the Commando Carrier HMS Albion in September. The last time that a Wessex attack helicopter saw combat was during the Falklands War – as illustrated by this painting (The attack by Lieutenant P C Manley and POA J A Balls in their Wessex on Port Stanley on 11 June 1982):
http://www.yellowairplane.com/Advent...Wessex-SSS.jpg
Source: Warpaint No. 65: Westland Wessex