nothing in that link mentions Rangers or Celtic being in favour it. Last time something similar came up a few years ago Celtic were pretty dismissive (and Rangers i think were in Scottish Division Naughty step so stayed out of it). i gained the impression that there more people in England...
English dont want it, Scots dont want it, Welsh, well they already have it, and the Northern Irish aren't large enough to matter.
only a few pundits and maybe businessmen ever advocate it, i dont recall Ranger/Celtic themselves being interested and no one else would gain anything.