Direct Paris Est - Strasbourg morning TGVs depart at 06:20, 07:14, 08:22, and 11:18. With such frequent service, the trains are not likely to sell out a week in advance. In fact, on a weekday they're not likely to sell out at all.
However, SNCF limits the number of seats it allocates to passholders. If the quota is gone on the particular train you wish to take, you may need to make a reservation on a different one. If you can be flexible on which train you take, you should be okay waiting until you get to London to make the reservation. I assume that the London office of Rail Europe will charge you more for a reservation than you would have to pay if you bought it from SNCF in Paris, but at least you won't have to pay RE's shipping fee for having it mailed to your home.