If you can drive your sub there then sure.
Wouldn’t it be cool if Fiji (before it’s suspension) were a nuclear power?
Would you rather live in a wasteland full of radioactive rubble?
I’m gonna go out on a limb and guess that’s a rhetorical question.
Whilst UK may have been reduced to rubble. treaty obligations would require providing military support to our allies and you would expect this arrangement to be reciprocated if the situation with an ally that had been attacked.
If any attack came, I would expect orders to be transmitted to satellites and rebroadcast as a loop, the sub commander would be able to pick them up, heck I would be expecting that the undersea ultra long wavelength comms would do something similar.