I have had to do this a couple of times -
I rang NHS Direct & explained the situation -
http://www.nhsdirect.nhs.uk/
NHS Direct are a very good service -
08454647
Then I think I phoned the local hospital/casualty - who have emergency clinics; with a GP - then I had to find a chemist that was open that had the tablets in stock. It was a lot of messing about, but I did get it sorted. Ask them at NHS Direct what to do, & explain the situation. They are very helpful.
Let us know how you get on.
These days I have a repeat script that the chemist sorts out, when I pick up the meds they give me an appointment card of when to pick up the next lot, all I have to do is to collect them; they organise sorting them out so I don't run out over bank holidays & things. Some Chemists also run delivery services.