re: It's all a con
"All the UK Research Councils plus Wellcome have a OA funding mandate of deposit into UKPMC after 6 months."
That would be interesting if it were true, but actually it is false. It applies only to articles published in Life Sciences journals - most science (physics, chemistry, astronomy, geology, mathematics, computer science...) is not covered, neither I believe are pure humanities papers (which are the province of AHRC). Not all UK research councils fund life sciences research! Maybe 2 or 3 UK RCs are involved, but I think that more than half of RCUK's seven members are rather unlikely to be concerned with papers of interest to UKPMC.