Worth noting that UK nukes apparently don't have PALs - the crews of the UK Trident submarines have all they need to launch. Of course, being the UK the ultimate guidance on what to do if Radio 4 goes off air (thus civilisation ending) is in the form of hand written letters!
The Guardian reported in 2016 that the options are said to include: "Put yourself under the command of the United States, if it is still there", "Go to Australia", "Retaliate", or "Use your own judgement". The actual option chosen remains known only to the writer of the letter.
Well.. if there's any truth to that reporting whatsoever.. blimey....