According to this post, Heaven and Hell which features former Black Sabbath vocalist Ronnie James Dio are playing Download on the 12th of June joining Lamb Of God and God Forbid, the two bands that have already self confirmed.
Look for an official announcement on the Download website soon.
HEAVEN AND HELL — the band featuring BLACK SABBATH members Tony Iommi (guitar), Ronnie James Dio (vocals), Geezer Butler (bass) and Vinny Appice (drums) — has scheduled the following dates:
Jun. 12 – Donington, England, Download Festival