Why doesn't the Concorde have any wing fences?
The main purpose of the wing fence is to prevent the boundary layer thickening in the wing due to the spanwise flow observed in swept wing, as can be seen below.



Image from fuckyeahfluiddynamics.tumblr.com

This is the reason for having wing fence in aircraft such as Mig-25- to avert stall and improve stability. In case of highly swept wing like the Concorde, this is not required as the mode of generation of lift is different- using vortex lift.



Vortices over the Concorde wing, Separation in three-dimensional steady flow, ONERA

In fact, there is a significant cross flow, as can be seen in the following image:



Flow above the Concorde Wing, Separation in three-dimensional steady flow, ONERA

Due to this, the wing fences are not required in case of highly swept wings like Delta and Ogival wings (as in Concorde), where separation leads to vortex lift. Mig- 25 wings, though highly swept, was neither of these- the wing generated lift by conventional manner.