The three-step ejection
Verbally request they leave. Give reasonable time and a route. If they refuse or return, use reasonable force to remove them. Escalate to police if any of aggravated trespass, threats, damage or other criminality is present.
When trespass becomes criminal
Aggravated trespass (obstructing lawful activity), trespass on protected sites, byelaw offences on transport and Crown property, and Anti-Social Behaviour dispersal orders all shift the picture.
The commercial premises nuance
A shopping centre or pub is 'quasi-public' — you can be asked to leave for any lawful reason and refusal becomes trespass. But you can't use protected characteristics (Equality Act 2010) as the reason.