At Eurostar Ticket Office at London St. Pancras International. But due to limited seats for pass holders, it can be easily be book out. 

You can try, but I highly, highly, sincerely doubt IF they are even willing to consider your request there. For INside FRance-very best&easy is done locally at SNCF-machine, or if the fear to not go on long advance chosen train is unbearable to you: call SNCF on their /e/n extension-and hope&pray for a dedicated agent on the line.

DOMestic TGV trains on all major lines to/fro Paris run about hourly, so plenty of choice. Longer distance and esp. OVERtheborder from FR are much more expensive and also much harder to get