LH2000,
the "Helderberg" was indeed found with sonar, but the sonar was being towed near the ocean bottom at the end of a 9,000 meter long cable.
HMS Echo has a multibeam echosounder and a sidescan sonar, but I don't know if they can get that equipment down deep. At the very least, they'll want to map the ocean floor...a pre-requisite before sending any AUV down.