According to Royal Mail's 2012 publication All About Postmarks (pdf download here)
"Royal Mail only applies postmarks to genuine items of mail. This means that any item submitted for postmarking should conform in all respects with postal service conditions: it should be addressed and bear at least an amount of postage appropriate to the postal service being provided."On the subject of 'Special Handstamps' (which they acknowledge and we know are not always applied by hand):
"Special Handstamps, which are sometimes referred to as Special Postmarks, normally incorporate a pictorial design, and are used to mark particular events and anniversaries .... Customers may post covers [to the Special Handstamp Centres] for handstamping ... Care should be taken to ensure that it is made clear which handstamp is required. This can be done by marking on the outer envelope which contains your covers .... the reference number. "The guide also states that all special handstamps - originating in Royal Mail or sponsored by an organisation, company, or dealer will be publicised in the Postmark Bulletin and, by implication (this isn't specifically stated, will be applied to any suitable item of post which is sent to the SHC by anybody, ie it is not exclusive to the sponsor.
So step forward Royal Mail and Syd Barrett: anybody who thought that they might be able to service their own covers or cards with a Pink Floyd (or any other) stamp and have this 7 July 2016 Cambridge postmark applied to it is likely to be disappointed. I have been told that this will NOT appear in the Postmark Bulletin and will only be used to cancel the special Royal Mail Syd Barrett cover.
Update 8 July
In the Comments below, Adrian wrote:
This may be a technical argument, but if this handstamp is not advertised in the Bulletin for general use and it is not an operational (inkjet) mark, surely it's just a cachet applied on a private basis by Royal Mail, and the cover is not a first day (or any other type of commemorative) cover. Anyone could do the same, which makes it worthless as a collectible item.Which takes us to the "here's one I prepared earlier" department. I was prepared to sponsor a postmark for the Harry Potter stamps issued in 2007. However, because I was using my own (clipart) artwork depicting a (generic) wizard, and not the Warner Brothers provided artwork (which were mostly woodcuts and, quite frankly, dire) Royal Mail would not accept my money, and this postmark did not go ahead. I think it made a very good cover, though! Norvic FDCs with real postmarks here!