I don't know how it is in England, but we have emergency pharmacies here. I don't know how that would work though, since the other pharmacy has your prescription.
Otherwise I would agree, go to the ER, they should be able to give you enough for the weekend. Or at least tell you where you can get some.

Good luck, I have been there!! I moved and ran out before I could find a new psychiatrist. I made it for two days, then I had to call 911 and I spent the day in the ER, where they thought I was having a heart attack. NOT pretty. That was when I was on Effexor and Celexa if I remember that right. It was a few years ago.