I've heard this used in Britain (specifically England). Could someone explain to me what its meaning is?
This could be an variation on the more common phrase
"That ship's already sailed"
This generally refers to a missed opportunity. For example, if you were planning on applying for a job, and then heard that someone had already got it, you might say,
"Well, looks like that ship has already sailed".
Here the ship is a metaphor for the job, but more generally it is a metaphor for any kind of opportunity.
I would expect that the phrase
"That ship never left the port"
would have a fairly similar meaning. But perhaps instead of the opportunity going to someone else or you being too late to take advantage of it, it would refer to a situation where a seemingly promising opportunity is never taken advantage of or, as suggested by @WayfaringStranger, was held back by its own flaws.
Yes. A ship that never left port is used to address a variety of things. A person who has never moved out of their comfort zone, doesnot venture forth, has not weathered life, hasnot left home, has not experienced life beyond their immediate existence, hasn't 'sailed" - can refer to sex, relationships, etc. A ship that has never left port usually never will. Could be aperson that will grow old with Mimmy and never marry, etc