I'd quite like double shields for British Islands (ENG, IMN, JEY, NIR, SCT, WLS) for routes with both primary and non-primary sections.
It'll be awkward though - I've got to go through gbna, imna, jeya, jeyb and nira and put them into three categories - primary, non-primary, both - and then the primary routes have yellow-on-green cartouches, the non-primary routes have black-on-white cartouches, and the both have both (either alternating or not).
It's a bit like Tennessee - there's a system of numbers, and then, on top, there's a set of roads classed as primary with different shields, but the numbers don't change.