Unfortunately, we have to live in reality. Those are examples which have evolved with their own histories, but the historical development there is the important thing. In reality:
If a county (equivalent to state) decides that it wants to secede, there is going to be a war.
If a city decides that it wants to secede from an existing nation state, it is going to be stomped flat.
If a house decides that it wants to secede, it is going to be ignored (unless it starts messing with taxes, at which point see above).
In reality, it may be possible for Scotland and Wales to secede from the Union. They are countries; there is an existing point of division there, even if it is one that is deliberately misrepresented by nationalists on all sides. Doing so is likely to be a tangled and messy situation.
What often gets glossed over in the idea that everyone should be able to determine their own governance is that actually, the effects of deciding you don't want to play by the same rules as your neighbours any more isn't just a matter for you. If Scotland secedes, Scottish business needs to decide what to do about this; so does English business and the English government needs to decide how it is going to relate to this other, smaller state in its borders.
England has a preexisting claim to all the land in England. Britain has a claim to all the land in Britain. We can decide to renege on that and divvy it up again according to those national boundaries, but land doesn't (believe it or not) belong to the people who live on it. Land belongs to the bodies which claim it. In this case it is largely England which 'owns' the land that constitutes the British Isles.
You can't just decide that you're going to declare your 'ownership' of a city or a house more important that England's ownership of it. England has a claim stretching back hundreds of years into the past and with an unspoken pact to continue to protect that land hundreds of years into the future.
Nations are historical constructs, and tend to be created only when other nations cease to exist and can no longer enforce their claim over the land that they define as theirs. I sincerely doubt that 'England' is going anywhere any time soon, even if 'Britain' does, so aside from all the logistical points as to why arguing for devolution down to such small levels is ridiculous, there's the practical point that they will fucking kill you before they let that idea get any traction.
