Any warranty offered above the legal minimum is done so because EVGA have faith in their cards and thus Brexit shouldn't have any bearing on what is offered outside of the legal minimum.
If you purchase a extended warranty then that is exactly what you get.
The UK gov will likely mirror the EU warrantys unless they make them longer which is doubtful. When it comes to consumer protections then things should likely remain unchanged.
 
All of the above is my own speculation and not based on any sources (hence the use of the words "shouldn't and likely")