The original use of this term refers to, I believe, the Tower of London - the prison for a number of high profile prisoners during Britain's history. But there is another tower - the Clock Tower which contains 'Big Ben' and is often referred to by that name. In the lower third of the clock tower is a room which was used as a cell for holding MPs committed into the custody of the Serjeant at Arms of the House of Commons.The House still retains the power to imprison. In theory an MP committed by the House could be detained in the clock tower until the end of a session (could be a long time - a person committed on the first day of the 2005-06 session would have entered in May 2005 and been released on 8th November 2006) - in practice the rule was that the person was held until both Houses had finished their business that day.
The last MP committed to the cell in the Clock Tower was Charles Bradlaugh, who spent the night of June 23rd 1880 there.
"(Bradlaugh) was informed by the Speaker that the House had decided against him and that he was authorised by the House to request him to withdraw. This Bradlaugh refused to do and repeated his refusal after the House had formally voted to exclude him. The Speaker ordered him into the custody of The Sergeant at Arms, Captain Gossett, who assisted by Police Inspector Denning took him to The Prison Room. There he remained in the charge of a Doorkeeper and PC 162 A McKay, until he was released at 5.30pm the following day. His imprisonment ended following Sir Stafford Northcote moving successfully for his discharge. This, however, was not prompted by sympathy as Northcote was a firm opponent of Bradlaugh and a prime mover in most of the action the House subsequently took against him. Evidently he considered that Bradlaugh at large to disrupt the House would suit his case better than Bradlaugh confined as a prisoner of conscience."
