Silent Service: The Nuclear Submarine in the Grand Line

In 1985, a Royal Navy Submarine known as HMS Chariot (S47) sets off from Faslane to patrol in the Soviet Union's home waters. This Churchill-class submarine is capable of being a threat to any enemy vessel on its path. Especially when one of the submarines of this class sank ARA General Belgrano during the Falklands War. It's arsenal included Tigerfish torpedoes with the improved variant being the majority stored onboard, a decent amount of Mark VIII Torpedoes onboard, and some Harpoon Missiles.

Unfortunately, the crew and their vessel would go missing due to unknown circumstances. And to ensure any issues (either internationally or within borders) don't arise regarding the topic of a missing NATO submarine, the disappearance of HMS Chariot would be heavily covered up by the British Government.

Now, where are they?

Somewhere, in another world far from ours. There are two sides of the Grand Line: The Pirates on one side, the oppressive World Government on the opposite end, and the poor civilians between the middle.

And now, the finest of Britain's "Silent Service" is now caught up between the two.

How can a vessel, more advanced than pretty much most ships in the Grand Line, survive in a very different world? With nothing but limited resources, a small manpower, and a submarine powered by nuclear energy.

(To keep things short, pretty much everything except the OCs of this fanfic aren't mine. If any copyright issue arises from this, I'll be happy to take it down to avoid any lawsuit of some sort.)