Nelson Mandela was thought a terrorist by the Tory right wing.
His actions were reasonable under the circumstances of a majority African population oppressed by white rulers. (Yes, I was around then and many like me thought him to be a guerrilla, not a terrorist)
He didn't cut off heads, eat human body parts, commit genocide, mass rape or slavery.
Islamic Jihad and their aims to conquer other lands to achieve the old Caliphate dream are a complete quantum leap in horror and severity, Also, no comparison to terror groups like Baader-Meinhof or the IRA.
Early in WW2, Parliament passed legislation to allow the Home Secretary to intern anyone judged dangerous to the war, e.g. British Blackshirts like Mosely, German nationals.
British planes bombed German cities, causing massive civilian casualties - e.g. 100,000 in just one night over Dresden.
All this was perfectly legal.
However, we didn't put Germans in gas chambers, regardless of what they did, because that would be war crimes too.
We should follow international law to deal with British Jihadists and hardline Islam followers, not just close our eyes and hope they'll fade away.
If this law is now inadequate, then the USA, EU, Australia etc would be just as eager as we are to modify it and together we have the clout
We should act now, before the problem spins out of control, e.g. the USA or EU might impose punitive sanctions on the UK or force extreme measures, after a 9-11 scale atrocity on their soil committed by Jihadists from Britain.