Firearms are heavily regulated in the UK, especially handguns and automatic weapons, after several horrific shootings in the late 20th century. Long-guns, such as shotguns and rifles are also tightly controled, one or more licenses being required. One exception that there seems to be for this regulation is for British-flagged vessels. There are also a document which publicly available on armed guards for ships, but it is not clear to me if this includes otherwise illegal weapons.
In order to better explain question, here's a scenario - ACME shipping has a cargo ship that frequently travels round the Horn of Africa, a known high-risk area for piracy. ACME decide that in order to safeguard their ships, crew and cargo, they must have a contingent of armed guards on board each ship. What firearms would these guards be allowed, under British law, to carry? (Only Rifles and Shotguns, or could these guards also be authorised by the Secretary of State to carry Assault rifles and handguns as well?)