I had a double pole isolator (similar to the one in the photo) installed at a property recently. I contacted the electricity supplier (British Gas) and ordered it. Their "meter team" phone me up and tell me they're from South East Power Networks (part of UK Power Networks) to book the appointment. The engineer arrives, installs the DP Isolator on their meter board, after the meter, connect up the tails and off they go. So, in my view, whoever installs that item, irrespective of who owns it, is responsible for a failure if a failure was to now occur. The only exception would be if another engineer, e.g. an electrician replacing a new consumer unit, opened the DP isolator, to put in new tails to the consumer unit, and may not have done them up tight enough again. If there has been no work on this DP isolator in the photo, then whoever installed it is (most probably) responsible. But of course then there's mechanical failure etc etc.