Yes, Sea Urchin was developed by British Aerospace in the early 1980s. It was 21-inches in diameter and was meant for use from submarines, surface ships, and aircraft. It came in a variety of warhead weights (250 kg, 500kg, and 750 kg) with a common sensing, fuzing, and arming system that used a combination of acoustic, magnetic, and pressure sensors.
So far as I can tell, Sea Urchin was never purchased. IIRC, the contemporary Stonefish from Marconi was procured as an exercise mine by the UK and as a warshot mine by Australia, with Chile apparently attempting to reverse-engineer it and failing. (BAe subsequently merged with Marconi, so Stonefish is now a BAE product.)