What I can't understand is why a vote in June 2016 is the fixed and unchangeably valid expression of the will of the people’s of the UK, while a vote in Spring 2018 would be a a mortal blow to democracy and an insult to the people of the UK.

There are two indeed rational bases for opposing such a vote. For committed Brexiters it would entail a risk that this time the vote might go against them. For MPs in constituencies where there was a strong vote for Brexit last time there is a risk that, if they supported a fresh vote, this might entail a risk of losing their seats in the next election, especially if it turned out that the will of the UK as a whole was to remain in the EU. But neither of these reasons are anything at all to do with democratic principles.