The Lords won't actually block the bill though for the constitutional reasons that 1 they can't use the Parliament Act now, they CD only do that btw 2nd and 3rd reading and 2 they are unelected. If they managed to stop it reaching the statute books the rightwing press could rightly say an unelected body had gone against the will of our elected representatives.
However it's still unclear if flights will leave as I cannot see how the bill without amendments satisfied the issues raised in the SC. So as soon as they try to get people on to planes there will be injunctions and we'll be back to square one. The SC saying the legislation is incompatible with our obligations under international law which it presumably will be is not equivalent to the Lords trying to delay it as it's literally their job to adjudicate on the legality of government actions (even if the press call them enemies of the people when they do this).
It sounds like there are practical issues with Rwanda wanting to delay too. I would be amazed if anyone ever goes to Rwanda.