The image shown, courtesy of Brian on the PostagelabelsUK blog shows usage at the PO near Caerphilly. The office CDS has been applied, as well as the Cardiff machine postmark for 20-IV-2010.
Brian has reported that some of the gold labels were mistakenly sent to POs in Kent. Some may have been used before instructions were sent to withdraw them, so keep your eyes open!
Updates:'Kent' referred to above turned out to be two branches in Orpington.
Bassett Avenue sub-PO in Bicester (branch 170137) also has them (before 8 May), as well as one in Norwich (before 14 May) and one in Stockton-on-Tees (posted 12 May).
In use 8-15 May at London 2010 Festival of Stamps PO - branch code 27508 London N1
20 May update: reported posted from Horsham, Surrey.
Also one from Sauchiehall Street, Glasgow G2 dated 12 May. Branch 362832.
22 May update: Lower Castlereagh (Belfast), branch 249704 - 1 May 2010. (Image to follow)