British nationalism only makes sense if Britain is considered to be a nation. Most people have been educated (conditioned) to think that it is. There lies the dichotomy in Wales and Welshness, where there appears to be a conflict of identity. Many people say they are both "Welsh" and "British". I would assert that this position is not tenable.
There is another point of view, which I trust Plaid holds, that Wales is a nation, just as Scotland and Ireland are nations. In that case Britain is not a nation but a grouping of nations in a single state. That is the reality and we must work from that, as it is the situation in which we find ourselves.
The point is that these nations, to cement their cultural, social and political identity, require nation status embodied in national institutions: a parliament, courts of law, and so on.
Either Wales is a nation or it is not. British nationalists, by their very belief, would deny that it is. For sure, true nationalists regard Wales (Cymru) as a nation and agree that the obvious course is towards independence and sovereignty within the European Union..
Anything short of this is a "cop-out", an evasion of responsibility. There are no degrees of nationalism. Either one is fully committed to the cause of freedom or one is a unionist, a British nationalist who denies the existence of other nations within its territory and the right of those nations to establish their own independent and sovereign institutions and become a nation within the European family of nations.