The Parliamentary Post Office cancel was and is applied to all mail collected there. Members of Parliament may use free franking on all official mail. It would also have the MP's signature or office stamp, in this case, Brian Gardiner. You, for example, could mail a letter there but it would require postage and it would get the same postmark.
This one is my "Free" example. It was political information mailed by my M.P. to an address in his riding.
