This is the Churchill AVRE (Armoured Vehicle Royal Engineers). It was a Churchill III or IV equipped with the Petard, a 290 mm Spigot mortar, throwing the 40 pound (18 kg) "Flying dustbin" with its 28 pound high explosive warhead. A weapon designed for the quick leveling of fortifications. It was designed after the Canadian failures at Dieppe and could also be equipped with numerous other attachments, such as mine flails, fascine rollers, explosive placers etc.
There are two of these fairly scarce variants in this photo. Note the British censor removed the unit identification.