In addition to machins fine post,
PHQ = Postal Head Quarters
Each card is numbered sequentially by Royal Mail as PHQ1, PHQ2, etc.. They can be found in the Stanley Gibbons Great Britain catalogues (well my 2005 Concise has them anyway) with the regular stamp and FDC listings for each stamp.
From SG front of catalogue boring fine print:
(Obverse means the picture side of the postcard and reverse means the side you usually write on.)