I've read about everything. I like to think that they were pre-IE and
it seems the most probable thing to me (as Celtic assimilation of the
British Islands was quite tardy) but I don't have any good clue.
A good link is this:
Pictish Nation. Check also
Wikipedia's article
(it even names genial and daring Basque Anarchist writer F. Krutwig,
who claimed they were Basques as did with Canarian Guanches as well, so
it seems open minded enough)
Btw, I'm going to write a stub article on Krutwig for Wikipedia, at least I've read most of his books.