Tell the bailiff to leave immediately and close the door.
If the bailiff refuses to leave, you can phone me and I can instantly determine the legal position, and cite chapter and verse to the bailiff and to police if they are present.
This can be over your telephone loudspeaker, or I can unexpectedly telephone the bailiff on his mobile while he is at your premises.
If you are in imminent danger of being defrauded or your safety is at risk, you should call police on 999 reporting a disturbance in progress. Do not use the word "bailiffs" in the call.
If the bailiff is not acting lawfully, then you can legally move the bailiff away from your home using reasonable force. If the bailiff resists, then the bailiff is the person that commits breach of the peace. See Foulkes v Chief Constable of Merseyside Police [1998] 3 All ER 705.
If the bailiff is acting lawfully, then deploy pay and reclaim and recover the money later through the courts. You may bring an action under paragraph 66 of Schedule 12 of the Tribunals, Courts and Enforcement Act 2007. If you are not a debtor, you may bring an action under section 3 of the Torts (Interference with Goods) Act 1977.
If a police officer assists a bailiff who is acting unlawfully, you can sue the police force.
This also applies if a police officer commits false arrest or falsely accuses you of "breach of the peace" by refusing to concede to an unlawful demand, or by refusing to open the door to let bailiffs in.