Nope, can't be worn before 5 Jun - direct cut from the DIN (available on Google)
The medal or medal ribbon may not be worn until entitlement has been confirmed. The medal ribbon and appropriate miniature may be worn from 7 Feb 12 (6 Feb is also the anniversary of the death of the late King George VI and is not a suitable date to be celebrated). The medal may not be worn prior to 5 Jun 12 (the date of the official Diamond Jubilee Thanksgiving Service).