Seasonal rituals are pretty well universal, and as old as civilisation (if not older!). Which seasonal rituals are practised, and what people think they're doing when they carrying them out, can change in as little as fifty miles or fifty years. But these discontinuities in practices and beliefs don't make the practices and beliefs any less worthwhile, and they don't make the needs they serve any less fundamental or real. I don't think there's much point in looking for pre-Christian cultural survivals, but more importantly I don't think there's any need - a 100% Christian spring ritual does the same work as a pagan one (if there is such a thing).

As for the point of the OP, ever since people started studying folklore a whole body of folklore-about-folklore has grown up, often stressing concepts of the Ancient and the Unchanging. So when people took a hobby-horse from door to door at New Year, that wasn't just a rural custom, coupled with a contemporary broadside ballad in a spirit of creative improvisation - it was Sleipnir, steed of Odin, who had obviously been commemorated in Yorkshire since before the priests got there. Part of the belief in folklore as unchanging is projecting the way we think about it now back on to the ancient past. In the case of the 'thinning veil', it works well for us as a metaphor, but we can be pretty sure it's not a description of what the Celts believed. Firstly, nobody talked in those terms before 1978; secondly, when people did start using that image, they were harking back to nineteenth- and twentieth-century ideas about reality and the spirit world being separated by a 'veil' - which in turn were based on a misreading of the King James Bible (the 'veil' of the temple was a pretty solid curtain).

Folklore about folklore keeps changing, but never reflects on it - because one of the key beliefs is that whatever we believe now is true, so it must be the same thing that earlier folklorists believed (not to mention the folk who had the lore in the first place). Ley lines are a great example. Alfred Watkins believed he'd discovered a network of tracks, which he called 'leys' ("the old straight track" indeed): not lines of power, not straight lines connecting high points on the map, but paths that the ancients used to walk down. Nobody now believes that; people who talk about "ley lines" now are talking about something very different from Watkins' original vision. They've entered the folklore of folklore, becoming something people believe in because other people have believed in them.