The British government have a strong, well-established (since ~1950), and arguably inflexible attitude towards "right to self-determination".

Meaning: the people who live in a place get to decide who governs them and their society.

People in Gibraltar, The Falklands, the Channel Islands, the Isle of Man and Northern Ireland all have different wants, needs and asks of the UK, and the UK honours all of them. There are other overseas territories with different relationships, plus the Commonwealth too.

If that ask changes (and in Northern Ireland it likely will in our lifetimes, the others less so), the governance will change.

Sometimes there is an ask for a closer relationship through the British Commonwealth (such as South Africa). Sometimes there is an ask for the UK to go away, thanks but no thanks (such as Malta). Each relationship is handled individually, but through a lens of self-determination. That's the priority in the FCO, in Parliament, in UK media. If a tiny nation wants to be British, the UK will go to war over it despite it making little sense (Falklands). If a tiny nation wants independence despite tactical advantage for UK to keep it, independence will be fought for (Malta happened, Diego Garcia is a WIP - watch this space). Where there is division (Northern Ireland), the majority view is observed, but with democratic and cultural structures created to try and make sure minority views have a voice in that governance.

That said, there is a caveat: observance of treaties tend to over-ride local preference in some cases, so if there is a legal argument to ignore the wishes of the locals, those wishes may be ignored: Hong Kong is the most prominent example of this in recent times (locals seemed to want to stay British, China said the 100-year agreement was up and there'd be no renewal, end of, so China it became).

Diego Garcia is another example, which has got messy because of the Whitehouse not understanding the UK's perverse inclination towards local democracy and the right to self-determination (see also non-UK entities the Whitehouse has not understood well: Greenland, absurd noises about Canada, and so on).

When able, the UK has consistently been committed to restoring governance to a local population's preferred model peacefully since ~1950 (India being the last real mess), and if the people of Gibraltar want to be governed by the Spanish, they'd be governed by the Spanish within ~2-3 years.

The idea that local people should have no say in this because "it's obvious" who "they belong to", is the colonialist notion here. A land isn't about geography. It's about people. It took a long time for the UK to understand this. Eventually they did. Most European colonial powers did. Others are still trying to catch up, it seems.