It will always be substantial as long as a high percentage of "British" Muslims believe terrorist attacks against their current nation of residence are justified. Welcome Islamists into one's nation with open, naive, tolerant, multi-cultural arms and one should expect to get terrorized. The disease of liberalism has now forced Western intel agencies and police forces to attempt to do the near-impossible.
"I have to be honest and say on the basis of what we know, there could be others out there...so the threat of terrorist attack in the UK is still very substantial."...
Dr. Reid is a master of understatement.