[2][3] The class was ordered under political and Treasury pressure for a relatively cheap, yet modern, general purpose escort vessel which would be attractive to governments and officers of South America and Australasia: the traditional export markets of British shipyards.

[citation needed] The development of Vosper's own export designs, the Mk 5 for Iran and the Mk 7 for Libya, increased the pressure on the Admiralty to accept this line of naval development, which seemed to offer a cheap export frigate with a range of 6,000 nautical miles (11,000 km; 6,900 mi), a top speed of 37 knots (69 km/h; 43 mph), a superficially good armament of the new Mark 8 4.5-inch (113 mm) gun, facilities for a Westland Wasp helicopter, anti-ship missiles and two triple lightweight Seacat missile launchers.

When plans for the new Libyan frigate Dat Assawari were finalised in 1968, the Admiralty board accepted that its paper specifications were unanswerable[4] and they would have to allow the shipyards to develop a low cost fill in anti-submarine warfare and general purpose version for the RN that would be stretched and fully gas turbine-powered rather than combined diesel and gas like the Mk 5 and Mk 7.

In reality, it was a much more difficult design, with the lack of heavy propulsion machinery low in the hull to balance the top weight of bulky superstructure.

The fitting of Tyne gas turbines for cruising, instead of the diesels used in the Iranian and Libyan versions, meant fuel consumption and cost would be high, which was a tremendous problem for the Royal Navy in the early 1980s when the austerity of early Thatcherism cut the Royal Navy fuel allowance and meant that most frigates spent more time tied up, rather than at sea in 1980–1981; and despite the smaller crew, running costs of the Type 21 were ten percent higher than those of the Leanders.

[citation needed] At one stage, it was hoped to build a joint design that would meet both the Royal Navy's requirement for a low-cost Patrol Frigate and Australia's General Purpose Escort requirement, with discussions between the two navies beginning in 1967,[5] with Australia, who hoped to build a series of Type 21s in Australian shipyards, part-funding design work on the proposal.

[11] As delivered, the Type 21s were armed with a single 4.5 inch Mark 8 naval gun forward, and a four-round launcher for the SeaCat surface-to-air missile aft.

The Exocets were located in two pairs and the missiles would deploy across the ship and clear the opposite side of the vessel to their launchers in flight.

The design error in weight and stability had more than wiped out the margins that had been intended for replacing Seacat by the more capable Seawolf and the Type 2016 bow sonar.

[13] The ships lacked the margins of weight and space customary to in-house Royal Navy warship designs for major modernization.

[13][14] Five modernisation proposals for the Type 21s were considered by the Royal Navy[13] but rejected by 1979, when it was 'reluctantly' decided not to modernize the class,[13] and it was estimated that they would be laid up by 1988.

After the Falklands War, two more 20 mm Oerlikon guns were mounted on some ships of the class, one each side of the hangar, to provide extra close-in armament.

As shore bombardment platforms and in lethal, accurate gunfire support for the Royal Marines and British Army landing at San Carlos, they were superb, pinning down any possibility of Argentine army counterattack,[18] but they remained shallow water surface fighting ships, designed for Vosper's export market to provide nations like Libya and Iran with the firepower to replace the United States/UK as western supporting stabilisers under the Kissinger/Healy strategy.

The ratings' accommodation was also improved, with four-man sleeping berths leading off from the communal mess deck; again, far better than those of the Type 42 destroyer.

[21] They were heavily involved, performing extensive shore-bombardment missions and providing anti-submarine and anti-aircraft duties for the task force.

On 10 May, HMS Alacrity and Arrow probed through Falkland Sound at night searching for minefields that might have impeded landings and operations, almost as expendable hulls[citation needed].