Although it is not officially a parish church, St Margaret's has long been proud to be known as 'the parish church of the House of Commons'. The close connection between the church and the Commons dates from Palm Sunday, 17 April 1614 when the whole House assembled in the church and took Holy Communion together. This formal custom continued for about fifty years, eventually to be replaced by other parliamentary religious observances. Recently, the custom of Members of Parliament receiving Holy Communion together has been revived in a more informal way. Also, many 'parliamentary' baptisms, weddings and services of thanksgiving are held here.

The crowned portcullis, the symbol of Parliament, is found throughout the church — for example, on the kneelers, the curtains behind the choir stalls, and on the red dors on the north side of the church. It serves as a reminder of the close relationship between St Margaret's and the House of Commons.

Since 1681 a pew has been set aside for the Speaker of the House of Commons. The Speaker's Pew is at the front on the south side and has the crowned portcullis emblem of Parliament carved on the Pew end. The Rt Hon Michael Martin MP, Speaker of the House of Commons, is pictured here with Canon Robert Wright Sub-Dean, and Rector of St Margaret’s and Speaker’s Chaplain.