The House of Commons has a line on the carpet which you're not allowed to cross as it's seen as being too aggressive towards the other side of the chamber. This is where the term "to cross the line" comes from.
Similarly, a new vote has to pass two stages, a vote in the House of Commons and then in the House of Lords. It's extremely unlikely for a law to pass the first vote stage and not the other, and once it has passed the House of Commons the papers are carried to the House of Lords in a bag. This is where the term "it's in the bag" (ie relatively certain) comes from.
Probably missed some factual details but you get the gist