A displaced residential occupier, a protected intending occupier or a person acting on their behalf can use reasonable force to secure entry to the property.
However, with regards to a protected intending occupier or a person acting on their behalf before entry can be secured, they must obtain a written declaration from a commissioner of oaths (usually certain more senior solicitors). Once this has been obtained then entry can be gained. If the squatters refuse to leave then they commit an offence and the police should be called to deal with the matter.
Any person using excessive force could leave themselves open to criminal and civil proceedings.
Outside the above provisions, a landlord/owner etc. cannot force entry into property where there are squatters whilst someone is inside. However, they can get a court order fairly easily and deal with the matter by that means.