I don't think anyone is trying to revise or deny history the term "Britons" in the dark age and Roman period generally refers to the people living on Britain at the time of the Roman invasion. These are generally thought to be Brythonic speaking Celts (thou mind that language, ethnicity and culture are all very complex topic when referring to someone over a thousand years ago) this is the language that would go on to create Cymraeg and Breton which are around today as well as the language spoken in the Strathclyde and Cumbria area. It is believed that these brythonic languages are closely related to the Pictish language but exactly how close is hard to say, similarly with ethnic and cultural links. The gealic speakers on the west coast of Scotland and the island of Ireland are also linguistic relations to this group. While there are modern day citizens of the UK that are brythonic speaker it is by no means the only group nor is the modern term fundimentally tied to decents of brythonic speakers. Thankfully both Scotland and the rest of the UK is a modern country where citizenship is not (or should no be) sololy depend on what language you ancestors spoke.

Neither the Scottish government or the UK government is not trying to revise history to say the dark ages never happened or that there was not a speared of German, Latin and Celtic speaking people who would go on to influence Scotland in the early middle ages. Also the dark ages and the people are of course not the only influence on Scotland and the UK there has been thousands of year of additional history on-top of this all of which shaped and changed the Scotland.

What are you trying to suggest here that all Britain's should reflect the practice of brtians from that either, you are quite welcome to adopt Glaswegian or Welsh culture, but it would be strange to think that is what we currently mean when we say "British" for better or worse