Little inside to this.
This was an APPG Event which is like an American Congressional caucus which is non partisan that holds events for a specific cause such as the APPG for the Holy See.

Clearly this was the APPG for British Muslims which did a prayer event not in any of the chambers but rather one of the spare rooms such as one of the parliamentary cafes or committee rooms not in use that held such an event.

While not ideal this APPG event , you would happy to know Parliament has its own Traditional Ad Oritem Chapel Consecrated and used prior to the Reformation called St Mary’s Undercroft. There is still a TLM Community that does the Holy Mass (1962) sporadically.

https://www.stjosephsbedford.org/mass-in-st-mary-undercroft-in-the-palace-of-westminster/

A description from the Parliament website reads:
The Chapel of St Mary Undercroft


The Chapel of St Mary Undercroft was completed by King Edward I in 1297, further developed under Edward II, and finally completed by Edward III in around 1365. While St Stephen's Chapel was the chapel of the Royal Family, the Court and the Royal Household worshipped at St Mary Undercroft.