MPs take an oath of loyalty to the Crown, representing the nation. The Lords (especially the modern Lords and Bishops) are appointed by the Crown on advice from

MPs take an oath of loyalty to the Crown, representing the nation. The Lords (especially the modern Lords and Bishops) are appointed by the Crown on advice from the Prime Minister, so other than the left over inherited nobility, who's power has been severely curtailed in modern times, no one in the Lords is beholden to the Crown for their position. If the monarch attempted to expand their power beyond their current minimal role in the UK, Parliament would exercise its supremacy. As for "without an electoral mandate", the current UK government is on it's 4th (or 5th? I lose count) Prime Minister because it holds the confidence of the Commons.