Plaid Cymru know in reality they can't seed from the union, not being a recognised nation in it is the real kicker... So the party knows in reality its aims are to protect the ideals of being Welsh within Britain,
Sinn Fein have had 100 years as being some form of a poetical force to have a united and independent Ireland. In 100 years they have achieved nether, killed a lot of innocent people and made complete idiots of them selves. Note ALL Irish independence in the Republic has a. been created by sensible people, not sinn fein idiots and b. is only symbolic independence anyway, the Irish economy is so heavily dependent on the UK they should really have been invited into the pound last time the Euro dropped.
Scotland know they can't realistically leave the Union, the SNP don't have the support and Scotland doesn't have the identity or economy to become a sovereign state....
So yeah Union, despite being the tyranny of evil with a terrible anthem and some views of uber backwards lack of change, works much better than "sovereign states" like France and other nonsense liberal states