Personally I'd recommend only two levels of Warlock.

What you really want out of it is Hex. As a champion, you want extra dice damage to make those crits matter, and Hex does exactly that. The rest of the spells can go towards utility if you're into that. Devil's Sight and Fiendish Vigor can't hurt, either.

Going too deep delays you too much, and frankly don't give too much for melee. If you're only interested in dice damage, other options would be War Cleric (Divine Favor) and Ranger (Hunter's Mark).