Second Stain, Can't speak for others but I have read your posts and yes I think it has been a quite serious consideration at several points over the past 3 decades or so however, I just can't see Celtic supporters accepting going to L1, maybe not even the Championship. I also can't see the league or PL managing to get enough votes for it to happen. That is even considering the increase in gates. After all just this pre-season I was at Sincil Bank watching the Imps play what had been widely publicised as a Celtic XI. Even Lennon on the Celtic sites had said it would be a reserve and youth mixture and none of the first team, yet Celtic had 2,500 in the ground (Lincoln's official figures can often appear to be reduced significantly, maybe for tax reasons

) and many more were unable to attend due to taking a tour of the Lincolnshire Constabulary Cells. I have no doubt they would sell their full allocation at EVERY game in the PL even if it were 10%.
On top of that, there's a fair chance that Scotland will be independant from the rest of Britain sometime soon. How can you have a foreign team playing in the English leagues? If that is seen as ok, then why not Ajax or some other big team playing in a small league? It shouldn't happen because it's immoral but I doubt that will be given a second thought if Celtic are considered for the English Leagues.
If I am thinking rightly (and I haven't researched) this would be that strange after all there are leagues with teams from independent states within them. Thinking Monaco as an example. If the Spanish independetnts get their way and Catalonia + Basque get their wish I doubt there would then be 3 leagues.
At present and yes it may well change near in the future, This is one country. No such thing as England, Wales, NI and Scotland apart from the name and the history. This is the United Kingdom and we have 4 separate leagues within that one country.
Like I say, Can't see it happening but can't really use the Independence argument with it.