Ahh the Churchill AVRE. This beast was designed to aid in the breaching of enemy defenses and was crewed by Royal engineers due to it's role. The standard Churchill gun was replaced with a petard mortar that fired 60lb shells. As you can see in the first shot, the barrel doesn't actually go into the turret for reloading. Apparently to reload, the turret swung over so that the barrel was above the co-driver's hatch so he could reach up and reload. This would have left his arms exposed and been quite a slow process.
The chassis was also modified slightly to allow the addition of a number of engineering tools like mine flails and fascine rolls.