Th UK Trident 2 missiles are literally the same missiles used by the US submarines operating in the Atlantic - both sets of submarines are supplied by a shared pool:
I had the same thought. They lovingly take Minutemen out of their silos and launch them after reassembly from Vandenburg AFB or someplace for a test. I’m pretty sure trident tests happen at sea.
https://en.wikipedia.org/wiki/UGM-133_Trident_II
However, the warheads on the UK missiles are designed and manufactured by the UK.