Disclaimer: I own nothing but my own OCs, should I create any for these stories.
When the aliens demand that UNIT trades them another hostage in exchange for Jo, everyone expects the Doctor to offer himself up. And he very nearly does - but everyone is surprised when Benton steps forward first, volunteering instead. The Doctor tries to talk him out of it, but the tall Sergeant stands firm and will not be swayed: "UNIT will get along just fine without me; they need you here, Doctor."
Reluctantly, the Doctor concedes to the Sergeant's logic - he is needed here, true; he just doesn't like to see others put themselves into the line of fire for him. He binds Benton's hands, as per the aliens' request, using the end of the rope as a leash of sorts.
When they arrive at the aliens' spacecraft, the Doctor asks Benton again if he's sure about this. It's not too late, after all, to unbind him and offer the Doctor instead. He's sure he could take them down from the inside. Benton just shakes his head, saying his mind is made up: "It's my choice, Doctor, and I'll stand by it. You just worry about defusing that weapon of theirs." So saying, he exits Bessie as gracefully as he can (which is to say, not very well, given that he can't balance himself with his hands), and holds out the lead for the Doctor to take again.
When Jo is safely back with him (thankfully unharmed, though she looks exhausted), all he can do is watch as Benton is none-too-carefully hustled inside the ship, and hope that the Sergeant's trust in him isn't misplaced.