Register Office for a 6 people ceremony in London?

Hi Everyone,

We will have our big wedding in Spain next year, where my family lives.
But we want to do the civil/statutory ceremony with both ours parents here, in London on December. Because it is our anniversary, this years falls on Monday.
But we do not want to hire a bigger room at the register office for 6 people, it will be expensive and pointless. Does anyone know of any register office that does ceremony on Mondays and have small rooms or accept the witness plus two more people? Thank you for the help