Trident risks

The British Parliament has voted to renew the Countrys independent nuclear deterrant, Trident, the submarine launched missile system. The new weapons will come into service in 2030 and last for approximately thirty years at a total estimated cost of £30-40 billion plus running expenses of approx. £130b over the period. So far, so good if it keeps the Country safe against unknown risks in an ever more dangerous and unpredictable world. The question is: will a future enemy find a way to neutralise the British fleet by the time it comes into service.
Every submarine (and surface vessel) has a signature noise, a hum from the propulsion system, the engine or simply the way it moves through the water. Now, imagine that a future enemy designs a tethered mine apparatus fitted with a programmable release mechanism. These mines may already be in place, tethered to the sea bed in those areas thought to be favoured by the submarines such as the strait of Gibraltar. They are easily updated to recognise the characteristic hum of the UK's new subs, awaiting only the signal to become active in time of hostility.
As the submarine passes over the apparatus it recognises the vessel and releases the mine to home in on the target. The relatively inexpensive mine then destroys the sub with incalculable cost, not only of the hardware but the highly trained sailors who operate it. All that money and loss of life for nothing.
It is a chilling thought.