They are the ghost army of Westminster:

Hundreds of ex-MPs with the right to haunt Parliament even though their Commons careers are over.

More than 350 former MPs – enough to form the largest party if they grouped together – hold coveted passes to the Palace of Westminster.

Reported by the Mail Online. You can read the full story here. 

I am a journalist and author. I am a journalist at the UK edition of WIRED magazine. In 2015, my first book Freedom of Information: A Practical Guide for UK Journalists, was published. My second book Reed Hastings: Building Netflix, was published in March 2020. I created FOI Directory in 2012 and have maintained it in my spare time ever since.