I am having the same issue - it looks like the HP UEFI is 32bit.
I have Lenovo UEFI working perfectly well on the same 2008r2 server, so settings are OK - we both need to upgrade to 2012 which has support for 32bit UEFI.
Royal pain in the @&$£ though, I wish HP would at least acknowledge this, or document their UEFI implementation.