L106 Antwerp was not a Royal Naval vessel but was one of several RCLs (Ramped Landing Craft Logistic) operated by the Royal Corps of Transport (RCT) and its successor the Royal Logistic Corps (RLC) of the British Army. This photo must have been taken soon after it was built, possibly during trials, as it was one of two that went to the Falklands around this time and subsequently (in 1984) to Cyprus. See Ramped Landing Craft Logistic at the Army website