Theresa May is getting quite good at making up lies and excuses for her incompetence. And here she goes again -
Tpim stands for "terrorism prevention and investigation measure", a mini-control order - the real thing having been banned on human rights grounds. Terrorists subject to Tpims aren't under house arrest, but are required to wear an electronic tag. Being supplied by G4S (which made a mess of Olympics security and which fraudulently charges the Home Office for people who aren't even wearing a tag), they don't work properly and come off easily. The article continues -
"When the TPims order was obtained, Mr Mohamed was said to have received terrorist training in Somalia and fought on the front line in support of al-Shabab. Court documents also say he supported a UK-based network supporting terrorist-related activity in Somalia and had been involved in attack planning against Western interests in east Africa."
As a British citizen Mohamed can't be deported, and police didn't have enough evidence to nick him for anything - hence the lightweight control order. Theoretically, he could be stripped of his nationality under the Terrorism Act 2000, but what would he do then? Apply for asylum based on fear of extradition to Somalia, of course.