It's worse than that. The real history of the Budapest Memorandum is not known (or is conveniently ignored). If it was, Sir Jacob's argument would be instantly exposed as a canard. Even
@RnaudBertrand doesn't quite grasp it in his (excellent and worth reading) post below. The Budapest Memorandum *also* covered Belarus and Kazakhstan (who also gave up 'their' nuclear weapons, and who signed essentially identical, parallel documents at the same time as Ukraine signed its). All the documents contained a clause in which the USA, the UK and Russia promised "...to refrain from economic coercion designed to subordinate to their own interest the exercise by [Ukraine/Belarus/Kazakhstan] of the rights inherent in its sovereignty and thus to secure advantages of any kind."
When the United States imposed sanctions on Belarus in early 2013, Minsk complained bitterly to the US State Department that the sanctions directly contravened the Budapest Memorandum (rightly so, given any reasonable reading of the text possible). Washington's official response was that these sanctions didn't count. What the US State Department argued was that its policy to economically coerce Belarus to do what the US wanted was not in fact against the spirit of an agreement in which the US agreed "to refrain from economic coercion to...secure advantages of any kind" because, in this case, the US had unilaterally decided that this was to the benefit of Belarussians. (The Memorandum of course included no such caveats.) In case this argument was not sufficiently decisive, however, the US State Department also said that, as far as it was concerned, "the Memorandum is not legally binding."
I am not arguing that the US was the only guilty party here. But ask yourself this: If way back in 2013 the US believed it could break the Memorandum whenever it thought it was justified in doing so, and that the Memorandum was anyway not legally binding and so offered no legal constraints on US action, why do many British politicians (Sir Jacob is in the majority) contend that the Memorandum is a hard, legally binding, inviolable commitment to defend Ukraine? The answer, of course, is that it offers a patina of legality and moral force (one must fulfill one's promises!) to justify something that our elites wanted to do anyway -- hoping, of course, that nobody would bother reading the text or learning the history of the Budapest Memorandum.
And, of course, nobody who actually counts does read the Memorandum or learn its history in action! None many of the politicians in whose hands we place decisions of war and peace. They just think it sounds about right and that it suits their prejudices, so they repeat it rote. Not our Press, whose job it is in a free, democratic society to explore and parse such matters so they may present them in clear terms to the engaged citizen while holding the powerful to account. The Press, on the contrary -- increasingly credulous as they are, increasingly seeing themselves as activists rather than journalists as they do -- also swallow it whole and repeat the narrative rote. Sadly, despite
@TuckerCarlson's scepticism, he was not sufficiently prepared, so he was unable to make the above argument. Had he done so, he would have exposed Sir Jacob as ignorant or dishonest (one hopes the former, at which stage a man as honorable as I believe Sir Jacob is would have done the intellectually honest thing and reassessed his position on the matter).
So, I guess it's up to people here to look into these matters and try to present the truth to our tiny followings. Not ideal: I don't have a sub-editor. But what's left?