HMS Frost was a Vanguard-class Nuclear Submarine. It was used by the Royal Navy for all undersea operations in its area of operations, the North Sea.
When HMS Gartside passed through an anomaly in the middle of the North Sea leading to the Jurassic, the Admiral dispatched the Frost to the scene. He was unwilling to let this anomaly stay active, as with Russian, American and Chinese ships frequently passing through, one of them disappearing off the British coast could start World War III. He ordered the Frost to fire a nuclear torpedo at the anomaly, so it would be sealed by the EM pulse, despite the risks the detonation of a nuclear device would have to the timeline. It fired just as the Gartside emerged from the anomaly. With the torpedo heading towards them, the Gartside evaded the torpedo, though the nuke was heading for the anomaly. The weapon that was fired by the Frost was sealed in the anomaly, when Connor's torpedo worked in the way it was intended.
The Vanguard-class nuclear submarines were commissioned in the late 1990's as a replacement for the aging Upholder-class subs. They were put into service at the start of the 21st century, and remain in service around the world's oceans today. They are scheduled to be replaced in the early 2020's. In real life, there are no Vanguard-class submarines that share the same name as the one portrayed in Primeval.