Can bailiffs blast their car horns and yell "Hello" when I don't answer the door?

No

Anyone shouting or blasting a car horn may be guilty of a breach of the peace.^[1]

There is no legal requirement for anyone to open the door to unexpected callers, or to anyone wearing police-like body armour and boots.

If a police officer attending the scene of bailiffs says he is there to prevent a breach of the peace, - a police euphemism for tolerating enforcement crime, and the officer intends to put the bailiff in a class above the law, - the police officer himself commits an offence.^[2] A person may make a complaint against the police officer before a justice of the peace at a magistrates court, to report the police officer for the offence.^[3]

Nothing in enforcement regulations makes bailiffs exempt from criminal and civil liability for causing a breach of the peace. Sometimes, it can revoke enforcement,^[4] and the debtor can sue.^[5]

^{[1] Section 5 of the Public Order Act 1986
[2] Section 26 of the Criminal Justice and Courts Act 2015
[3] Section 1 of the Magistrates’ Courts Act 1980
[4] Regulation 10 of The Taking Control of Goods Regulations 2013
[5] Paragraph 66 of Schedule 12 of the Tribunals Courts and Enforcement Act 2007}