I'd be amazed if he had gone anywhere near her if he genuinely thought she was armed
In a nutshell. ^
I'm not police, but I'm pretty sure that had police been required to remove a protester, they would have firmly told her she had to leave. If she refused they would explain what law she was breaking, tell her she had to go, and, maybe, steer her by the elbow.
Also, from Liberty's handbook on protest:
(R.E Trespass)
Most landowners and their agents, typically security guards, avoid using force to remove trespassers who refuse to leave. The usual response is to call the police who generally encourage the trespasser to leave. Where the trespasser refuses the police may be able to justify arresting him/her if they suspect that their continuing presence there may lead to a breach of the peace.
It then goes on to talk about Trespass with the intention of intimidation ( eg hunt sabs) and that this is a criminal offence and carries a fine. Nowhere does it say that a bystander at an event has the right you grab a trespasser hard by the neck. Let alone police, and security.
Excessive force is a huge no no.
Mark Field is just a man who felt like attacking a woman and did so. His career is over.