The Main Bureau de Change is on the (right-hand (south) side of the central passage, opposite the Eurostar check-in.
This is on the east side of the main passage, the second unit to your left (south) of the Eurostar exit.
As you continue (south) from the central passage, you pass M&S, the Eurostar exit and the hotel booking office.
There's a pillar to either side of the entrance, the counter is in front of you.
ST PANCRAS Station
© 2009, Terry Robinson