Allow me to refresh the memories of the BBC. The Mirror report from the time of their arrest makes clear that, in fact, they were planning to plant a large car bomb. The report has this quote from police:
"We are convinced that if we had not acted swiftly a massive bomb, designed to kill and maim hundreds of innocent people, would have been set off in London, probably before the end of the year and possibly during the Christmas rush."
Also, it should be noted, that one of the defendants, Younes Tsouli, has been names in a case in America as having been sent information about targets in Washington DC. According to the local newspaper in the US he ran a website teaching terror tactics and called himself "Irhabi (Arabic for terrorist) 007".