It's been noted on this site that even if Parliament passes an extension, Parliament can't ask the EU for an extension directly themselves, that the law covering withholding the asking of Royal Assent for a bill is a bit vague, and it's been reported that Labour won't back a General Election, if Boris Johnson was to opt to either:
A) Refuse to give the bill for Royal Assent, or
B) Refuse to ask the EU for an extension (even if he passes the bill for Royal Assent)
What, if anything, could the opposition parties do about it within the timelimit, given they have no intention of calling or backing a General Election.