They have Tridents on land?
He wrote NUCLEAR deterrent, not “Trident deterrent”.
No. But they used to have land-based missiles. Not sure if they do now. I just googled around, and the Trident force seems to be the only point of contention when arguing about nuclear deterrent.
http://www.fas.org/nuke/guide/uk/doctrine/sdr06/index.html
IF so, then this was indeed a quarter of their entire nuclear deterrant at risk. Which is kind of scary.