"Boris Johnson adds to pressure on Germany to send tanks to Ukraine." How exactly? The Germans are going to pivot on foreign policy because an ex-British PM rolled into Kyiv unexpectedly? I mean, I'd like the Germans to change policy but I don't think this is the means.
The idea of making Boris Johnson special envoy to Ukraine is a deeply silly one and Sunak would be foolish to hand that to him. Appeasing Johnson would be a mistake - like a naughty little boy, he needs to be chastised for his stunts, not rewarded.
"Brexit benefits" fall into two distinct categories. Category one: those that aren't actually benefits (losing holiday pay, for example). Category two: those we didn't need to leave the EU to get (like helping Ukraine, which we could have still done as a member).