That's not actually correct. The missiles (the rocket part) are indeed leased from the US, and the maintenance every few years is thus done by them - which keeps the cost down. The warheads are the UKs.

A freshly serviced trident missile should be good for 5-7 years (although they currently get serviced more frequently).

If the UK falls out with the US then we have a number of years to either build our own replacement missile to put the warheads on or buy/lease a missile body off another country - France for example has several options in this space that could be easily adapted.

We are where we are because nobody could imagine the US going rogue like this, a failure of imagination for sure but our nukes are in fact in our control

https://ukdefencejournal.org.uk/heres-how-britains-nukes-are-operationally-independent/

https://www.chathamhouse.org/2025/03/uks-nuclear-deterrent-relies-us-support-there-are-no-other-easy-alternatives

https://ukdefencejournal.org.uk/no-america-doesnt-control-britains-nuclear-weapons/

Edit, tidied up the wording