But late last year, a select committee exercised these powers anyway: https://www.theguardian.com/technology/2018/nov/24/mps-seize...
What's not abundantly clear is if Parliament can actually still carry through its threat of imprisonment for contempt, if the recipient of the order decides to face down the Serjeant. It's possible that the courts would swiftly issue a writ of Habeas Corpus on the basis that contempt of Parliament is defunct.
It's also possible that the European Court of Human Rights would consider legislative proceedings for contempt to be a breach of the Convention, but as actions of Parliament are outside the reach of the Human Rights Act (which makes the Convention directly applicable in the UK) they have no way of enforcing the decision even if they made it - and it would take months at least to get to the front of the queue.