Litvinenko's killers knew where MI5 wasn't

A sizeable team of assassins apparently shadowed Alexander Litvinenko, the former KGB agent before he ingested a deadly dose of a radioactive substance, according to the Times of London.
So wouldn't that indicate that this hit squad was quite sure that MI5 or some other British security unit was not keeping a close watch on him? Considering how suspicious Russians -- and professional assassins in general -- tend to be concerning such things, it is unlikely they simply assumed that British watchers would not be present.
Or perhaps poisoning was seen as a means of bypassing any such surveillance. Yet, the killers seem to have had a pretty good understanding of what would be happening inside British security.