Standard Bearer with Last Stand as the only remaining model

You can overrun.

BRB p. 58: "If a unit charges into combat and, by the end of that round of close combat, all its enemies have been wiped out as the result of the combat (rather than the pursuit) the unit can choose to make a pursuit move, even with nobody left alive to pursue. This is an overrun move and represents the unit surging forwards, hungry to find more enemies to fight.

An overrun is essentially a special pursuit move. When making an overrun, the victorious unit moves 2D6" directly forwards, as if they were pursuing a fleeing enemy to their front All other rules governing pursuit moves, such as intervening units and terrain, apply to overruns."

Note also BRB Official Update Version 1.9, p.8: 

Q: If a unit charges into combat and, on the turn it charges, the last of the enemy unit it is fighting are removed as casualties due to Daemonic Instability, the Unstable special rule, Cornered Rats or as a result of a War Machine failing its Break test, does the charging unit get to make an Overrun move? (p58).
A: Yes.

Q: If I charge an enemy and they are wiped out before the Close Combat phase, can I choose to either Overrun or Reform From Victory? If I can, when does this occur? (p58)
A: Yes. This occurs at the start of the Close Combat phase before any blows have been struck.

My apologies - I somehow read over that. Quandoque bonus dormitat Fidelis. In that case, they can only reform.

BRB p. 54: "Of course, if one side has been completely wiped out in the fight, the other side is automatically the winner. In such cases the unit automatically restrains pursuit and reforms (as discussed on page 57)."