Sorry but I would disagree - it’s a very ambiguous question. For example the legend of King Arthur/Lancelot/Guinevere is equally (if not more) well known.
The UK did not exist in 700, whether CE or BCE, and Celtic tribes did not invade in the CE, they were largely conquered by Anglo-Saxons and others and remained in what later became Wales, Scotland, Cornwall and Ireland.
The Celtic tribes did not invade in 700. By that time they had been pushed towards the western fringes by the Angles, Saxons, etc. The next invaders were the Vikings.
I mean the modern-land territory