St Pancras is literally just across the road from King's Cross.
We had a bad moment when we discovered that Eurostar had lost the online record of our booking. Fortunately a very nice man came to the rescue after we wafted our hard copies at the desk.
Security was like at an airport but worse. You have to formally clear UK passport control, then formally clear French passport control. I found this very confusing because I thought we were going to to Belgium...
© (except the blatantly ripped-off bits) Random Bozo 2008