If you want a/the cheapest ticket, buy now. If you buy at the station, on the day it will, invariably, be (much) more expensive.
There are no overnight trains to London from Paris.
The earliest (on weekdays) leaves the Gard du Nord at 0643 arriving into St Pancras at 0759;
The latest leaves at 2113 and arrives at 2234.
There's lots of useful fare and timetable information here:
http://www.eurostar.com/dynamic/_SvB...CY=ESTAR&VT=EB