The BBC have done a decent job of explaining why it took as long as it did to get to this point
How Anjem Choudary's mouth was finally shut
http://www.bbc.co.uk/news/magazine-36979892
It's pretty much what I suspect many people thought, he knew the law well enough to just be on the legal side in what he said and did most of the time (rather like the Westborough Southern Babtists in the states), but the police were waiting for that slip.
Although I wish it hadn't taken so long to deal with him, I'm glad it was done within the existing laws and they didn't simply create a new law specifically for him, or deal with him in a special way - they took the long but sound route of getting him solidly under the law.
I wouldn't be surprised if he's put in semi isolation both to stop him spreading his brand of Islam in the jails, and because I suspect there are a lot of people who would love to have a go at him, and I don't want him to become a martyr in the eyes of his followers.