The St George tartan was designed to mark the tercentenary of the Act of Union (joining England and Scotland). Hence the Union Jack colours.
It makes clever use of the colours, suggesting the flag and especially the cross of St George.
There is also a St George check, intended largely for use as linings for clothing, which is mostly white with a red cross.
So far there doesn’t seem to be a Cumbria tartan, but I imagine it won’t be long before there is one.
Regards,
Mike