Are bailiffs allowed to call on their own?

Yes

An enforcement agent must have a valid enforcement certificate.^[1]

Anyone without a valid enforcement certificate may not attend unless he is accompanied by a person with a valid certificate.^[2]

^{[1] Section 63 of the Tribunals, Courts and Enforcement Act 2007
[2] Paragraph 44(1) of Schedule 12 to the Tribunals Courts and Enforcement Act 2007}