No.
I don't think you can book FIP fares online for Eurostar either unless things changed recently (someone will correct me if so I'm sure). Best bet is to come to St Pancras yourself and book in person. Price is £34.50 per person each way but this is quota controlled so may sell out quickly on popular trains (weekends in July/August for example). There are some agencies who can also book them for you subject to a booking fee (£10 per booking from International Rail which is the recommended one from RSTL if I recall correctly).
I believe for TGV trains there is a French SNCF number you can call to book the reservations on the phone (English speaking service) but can't recall it off the top of my head. You can also turn up at the station and book it at the ticket office during opening hours, but beware the queues at busy stations can be horrendous at busier times. The reservation costs €1.70 for trains classed as off-peak and €10 on peak services (based on demand not travel time), both in second class.