Neighbours said two Asian men had moved into one of the searched houses about a month ago but had kept very much to themselves.
The UK's "critical" status is one of five security threat levels.
The threat levels are:
Low - meaning an attack is unlikely
Moderate - an attack is possible, but not likely
Substantial - an attack is a strong possibility
Severe - an attack is highly likely
Critical - an attack is expected imminently
The level is set by the Joint Terrorism Analysis Centre (JTAC), which UK authorities created in June 2003.
The last time the level was raised to critical was last August, after police said they had foiled a plot to blow up flights between UK and the US.
"I don't remember seeing them at all," said Mae Gordon. "They were the only people around here you would never see."
Britain has seen an increase in attacks since it joined US forces in invading Iraq in 2003.
Analysts say the latest attacks may be designed to exert pressure on Britain to withdraw its troops from Iraq and Afghanistan.
Brown, who took over from Tony Blair on Wednesday, called a third meeting of Britain's Cobra committee, a national crisis response team, to discuss measures to handle his first major challenge in office.
Speaking on BBC television on Sunday, Brown warned the "fight against terrorism" would be drawn out.
"Irrespective of Iraq, irrespective of Afghanistan, irrespective of what is happening in different parts of the world, we have an international organisation trying to inflict the maximum damage on civilian life in pursuit of a terrorist cause that is totally unacceptable to most people," he said.
In response to the events in the UK, the White House announced that security was being stepped up at US airports, although a spokesman said there was "no indication of any specific or credible threat to the United States".
The developments came exactly a week before the second anniversary of the July 7, 2005 suicide bombings in London which killed 52 people during the morning rush-hour.