The UK's threat level was upgraded from "severe" to "critical" on May 23 following the Manchester bombing on May 22, in which a bomb exploded in the foyer of Manchester Arena moments after US pop star Ariana Grande left the stage.
Britain's terror alert level has not been raised.
A bucket packed with nails and explosives blew up on a crowded Underground train at London's Parsons Green station on September 15, injuring 30 people.
ISIS said a group "detachment" had been behind the bombing.
"We have made a significant arrest in our investigation this morning".
5 things to know — London train bombing
Scotland Yard says he was detained in the port area of Dover.
The area around the station has been evacuated, with specialist officers there securing the remains of the improvised device and ensuring it is stable.
"For this period, military personnel will replace police officers on guard duties at certain protected sites that are not accessible to the public", she said.
The news comes after police on Saturday arrested at 18-year-old man in Dover in connection with the rush hour attack.
The evacuation began at 1.40pm today, and the Met Police are being supported by Surrey Police. No one was killed in the incident.
Britain's terror threat level remains at critical - the highest level - meaning authorities believe another attack is imminent.
Sunbury is 10 miles west of Parsons Green, where the explosion occurred.