Careful ~ this is a classic case of a little knowledge being a dangerous thing.
Whilst a citizens arrest looks easy there are other factors such as when making the arrest the Judges Rules state that a "Proper Caution using the approved form of words" must be given to the arrested. It is also an offence to detain any person even if you are "arresting" them. You may also provide the arrested with the perfect excuse for them to later argue that (i) the arrest was unlawful or (ii)they have been wrongfully detained.
A further knock on effect could be that the citizen's arrest, deemed to have been wrongfully carried out, might be used by any half way competent brief to have any subsequent arrest, on charges, by the police to have the police case thrown out of court.
A formal complaint of wrongdoing by those you wish to accuse, if made to the police in the correct manner, leaves the police no alternative but to investigate the complaint and submit the details of their enquiries to the CPS for a decision on whether to prosecute.
Meanwhile making a citizens arrest takes you nowhere at all because having made the arrest, even in the correct manner, you MUST then hand the arrested person into the care of a police officer. You are obliged, having made the arrest, to contact the police immediately and provide them with all the details. You MAY NOT forcibly detain the arrested person. You may well, if you attempt this proposed course of action, find yourselves in the dock whilst those you seek to humiliate will be claiming damages against you. There are at least three if not four charges that the police could bring against you if you try this stunt, especially under their noses, at Westminster.
One of my tasks whilst working in the Manned Security Guarding Industry was the training of new Security Officers. This included two days in the classroom explaining in detail their powers etc. I do know what I am talking about.
Good luck ~ Hope you like porridge, you may find yourselves eating quite a lot of it. Get yourself some proper legal advice first.
My advice is to keep away from this one and go about it properly.