I used the list here: https://en.wikipedia.org/wiki/Counterforce#Counterforce_disa...

Britain uses Trident SLBMs, so should be capable of such an attack. However, further research reveals similar capabilities of the French M51 SLBM.

All in all I wouldn't give too much on that list of stated or probable strategies anyways. An apparent strategy and the stated conviction to stick to it is necessary for deterrence. But I'm not so sure of what will happen if push comes to shove. I guess every nuclear weapons state has a number of war plans fitting the various possible opponents and strategies, and which one is picked strongly depends on the current mood of the ruler when pressing the red button.