The United Kingdom of the Britons
This is an idea for a united Kingdom that encompasses the entire island of Great Britain and its outlying islands.
The Black and yellow is from the flag of St.David, the patron saint of Wales and of the Britons.
The White outline separating the yellow and black is from St.Piran's flag of Kernow (Cornwall).
The White Saltire is from St.Andrews flag and represents the Highlanders descended from the Picts.
So basically... a Celtic Britain!
The Kingdom of Gwynedd and Kernow, along with Cumbria and the other remaining celtic kingdoms that formed after the roman demise, manage to push back the anglo saxon invaders and eventually, in the 7th century , form a Kingdom that encompasses what we know as England and Wales, aswell as the lowlands of Scotland and Cornwall.
The descendants of the Pict's formed the Kingdom of Alba in the Scottish highlands and after some sort of Royal union (similar to James 6th/1st coming to England after Elizabeth 1st's death) the Kingdoms where united.
Still working on an Capital .-.