mikeb: at the risk of straying (albeit factually) into current affairs, the short answer to your question is "yes, but". The two principal buts are: it's commercially in the wrong place for Dover Straits traffic and how would you get the traffic to/from Dover and Cheriton from a statutory/"Customs" point of view - and, indeed, from a social/H&S one. It's not current government policy to mandate future UK "Customs" import or export clearance to be at a single physical location per 'route'; however, Manston is evidentially an asset that the state already has, as opposed to one that it might acquire or construct.
Mickleburgh: "Customs" clearance per se is not part of the UK/EU27 negotiations. In large part, the UK will have scope to shape its future inbound and outbound processes, although I understand that government policy is for them to be conformant with the Union Customs Code (UCC). Customs processes are largely an EU competence, not an individual Member State one, so the French definitely will have to comply with the UCC. It is their inbound processes (on our outbound traffic) that could well trigger Operation Stack, to which the current answer is "send the trucks to Manston". As Sentinel S4 may well recall, there are two mechanisms for getting road haulage across borders and 'playing Customs' later, eg. inland at the likes of Lenham ICD, or at destination in Eire and so on. These are: Common Transit and TIR - neither being an EU institution. The UK is currently covered by each, but the snag is, only by virtue of being a part of the EU, which is the "club member" of the mechanisms. The UK could apply to (re-)join the mechanisms in its own right, eg. Serbia, Turkey and the EFTA states are members of CT and basically everyone's a member of TIR, but I am unaware that it has done so yet.
I say "Customs" because clearance is not just about collecting statistics, collecting/securing any duties and taxes, and handing out any refunds - there is a whole raft of non-fiscal controls that the EU and the UK apply to goods to/from anyone who isn't in the EU. Plant health controls can usually be done inland, but controls on "meat/animal products" (the scope of which is surprisingly wide) typically take place at the border - ouch! These non-fiscal controls are probably proper to the Brussels negotiations, but I haven't heard a dicky-bird about them.
