Hilarious in Hindsight: The Federation at war with the Klingons, and one ship is crucial to the war effort? Sounds a lot like the first season of Star Trek: Discovery more than 45 years later. The Discovery even looks vaguely like the Lexington (which was called a "heavy Research/Battle Cruiser") from EGA Trek!
The Sinclair / ICL port to the ZX81 files off any Paramount trademarks — the game becomes "Star Trail", the starship becomes the Endeavour, and so forth.
Same goes for the EGA Trek line — Klingons become "Mongols", Romulans become "Vandals", the Federation becomes the "Union", the Enterprise gets a visual change and becomes the Lexington, dilithium becomes "energium", phasers become "lasers", photon torpedoes become "energy" torpedoes, and so forth. Less prominent details, like subsystem names or planet types, remain the same.
On the upside, you can also obtain these by raiding Mongol supply bases and supply ships, and then turning them on their former owners, typically for a One-Hit Kill, or a One-Hit Polykill on tightly grouped enemies — assuming it doesn't fail to detonate, that is. And then, they still tend to deal damage to the player's vessel, even when used from a distance.
That One Rule: Aiming torpedoes. The makers of the Star Trek III.5 variant of the game actually sold aiming charts, so that you could figure out exactly what bearing to fire your torpedoes at, depending on the sector the Klingon ship was in. (Averted in EGA Trek, which allows the player to specify coordinates to fire the torpedoes at, although they can still miss.)