This is now live, as reported in many places.
UKPMC mirrors the journal archive content of PubMed Central but will contain manuscripts submitted by people mandated to do so by the conditions of their grant. Several grant awarding bodies, among them the MRC and the Wellcome Trust, are mandating people who they fund to place the final manuscript of their work into UKPMC.
Go to UKPMC itself.
Read more from the Wellcome Trust or from JISC.