The first reason for the British to build a submarine is to catch Captain Nemo, while he is under the sea. The mechanical tentacles could be maneuvered to catch and put a hold on the unsuspecting Nautilus
A vessel like the Lusca could also be brought into play against other enemies of the realm. The physiological effect on Britain’s enemies would be great. Think of the terror the vessel like this would stir among the crew from a vessel being attacked by what looks like a giant squid?