Lots of parts of the UK were once nations.  How far back do you want to go?  There was once Wessex, Mercia, Northumbria among many, many others.  Usually these various parts were taken over by others, usually with a war or two on the way.

There are big differences in culture, accent and scenery between almost any parts of the UK, but I don't think anyone seriously thinks there should be a Republic of Yorkshire or Surrey.

If Scotland got independence then what if the Highlands wants to split from the Lowlands?  What about the Republic of Orkney?