Have our elected representatives behaved poorly? Absolutely. The management of Brexit under the premiership of Theresa May was negligent, if not downright disingenuous. Three years of soft soap, culminating in a withdrawal agreement that was impossible to endorse. However, I have to correct your suggestion that, by not following the withdrawal agreement, the UK government is "welshing on that agreement". Theresa May negotiated the terms, but these were always subject to parliamentary consensus, which has failed on three occasions.
It is rarely referred to - surprisingly - but Theresa May agreed the final withdrawal agreement terms without reference to her secretary of state for Brexit. He was blindsided completely. Sound like a stich up by a PM to hinder the process? It does to me.
So, yes, our politicians have, over the past 3 years, behaved very poorly.
However, the new PM seems to at least be seriously looking for a withdrawal deal that can be endorsed. Recent concessionary statements by Merkel and Macron indicate that the negotiating strategy to leave "no deal" open, could actually lead to "a deal".
The problem with the plotting Remainers is that their fig leaves are dropping. They all claim to be willing to honour the referendum result to leave the EU, but will only countenance doing so with a deal. But - the only deal offered so far is the withdrawal terms negotiated by Theresa May, and parliament has already voted this down three times.
By trying to remove the threat of "no deal", but with no consensus proposal as to what deal they would accept, the Remainer contingent are ensuring that the terms the EU offer on withdrawal will not improve. Since those terms are not supported by parliament, the only conclusion one can draw is that the actual Remainer intentions are not leaving with a deal, but not leaving at all.
In this context, and noting that the current withdrawal agreement is not binding on parliament, the charge of undemocratic behaviour can only really be levelled at the Remainers.
As regards proroguing of parliament, the anger here is "confected". In 1997 John Major used the valid constitutional tool of suspending parliament just before the general election in order to keep an investigation over government corruption (the "cash for questions" debacle) out of the public eye until after that election. There was no hysteria then, no confected anger, so there does not deserve to be any now.
In short - three years of Theresa May were wholly ineffective, to the point of negligence. Whatever one might say of the current PM, he is at least looking to find a consensus withdrawal deal.