"Foreign dynasty needs stronger local branding, commissions elaborate content tying itself to beloved national mythology" could almost be a communications strategy written today. Merlin's Cave is just an unusually expensive 18th-century version of it.

If you like Arthur, you may be surprised that some scholars think he wasn't English, but rather he was Cymro/Welsh: https://www.wales.com/about/history-and-heritage/welsh-tradi...

Wht do you mean by some surely it is all who think he is non English,

The Angles – ancestors of the English were the people whom he was fighting against.

Now was Arthur Roman or Celtic or Welsh etc is up to debate.