Thanks for your response Aswan, I've just received a response from Nestle Consumer Services as follows:
"Optifast® is recommended for use under close medical supervision.
Unfortunately, Optifast is not available over the counter in the UK and Ireland. In order to obtain Optifast, you will require a private prescription from you GP, which must be paid for in full on collection from a pharmacy, or a specific ordering card to complete. This order card can only be obtained from a Bariatric Consultant or Dietitian. The card is then sent, along with payment, onto our fulfilment agency who will dispatch the Optifast to you directly."
I guess I'll just have to bring enough for 4 months with me.