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 , a top speed of , a superficially good armament of the new Mark 8 gun, facilities for a Westland Wasp helicopter, anti-ship missiles and two triple lightweight Seacat missile launchers. When plans for the new were finalised in 1968, the Admiralty board accepted that its paper specifications were unanswerable 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 CODAG like the Mk 5 and Mk 7. In reality, it was a much more difficult design, with the RN requiring the extra internal weight of the Computer Assisted Action Information System (CAAIS) computer command systems and the lack of heavy diesels or a steam plant low in the hull to balance the heavy top weight of CAAIS. 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 "Leander"s.