If it did turn out that the government were responsible for Salisbury, it is quite possible that under standing order 25 that parliament could vote for a state of emergency, get rid of Boris Johnson and thus guarantee an extension. The state of emergency would be declared because the UK is at risk of terrorist attacks from its own intelligence services. It is the “coventry solution” in that in dong so one damages the intelligence services as opposed to the UK as a whole. Except of course that SIS and GCHQ are not as well thought of as Coventry was. Margate perhaps
Furthermore if you were an ardent remainer in parliament, claiming to be concerned about the effects of no deal upon the country and if there was such information, why would you not proceed on this basis in order to stop brexit? Would it not rather discredit your alledged concern about the country if you were not to proceed?