PA, I didn't think Trident was being replaced? I thought it was the"just" the submarines at the end of their hull lives?
One imagines that it will be a similar arrangement to that at present, with missiles being taken from a pool common with the USN. UGM-133 is undergoing a life extension programme (D5LEP) to take it up to ~2042. Whatever replaces it is certain to fit the same launch tubes, given that the Ohio Class boats are due to be replaced from around 2031 onwards.