Here's a sample charge from the police press release [0]
Preparation for terrorism. Between 31 December 2015 and 22 September 2016 Samata Ullah, with the intention of assisting another or others to commit acts of terrorism, engaged in conduct in preparation for giving effect to his intention namely, by researching an encryption programme, developing an encrypted version of his blog site and publishing the instructions around the use of programme on his blog site. Contrary to section 5 Terrorism Act 2006 [1].
Here's the Act [1] :
A person commits an offence if, with the intention of— (a)committing acts of terrorism, or (b)assisting another to commit such acts, he engages in any conduct in preparation for giving effect to his intention.
In this case encryption is the conduct. It is not the conduct which gives rise to the offence, it's the intent. The actual nature of the conduct is legally irrelevant. He could just as easily have been charged with the same offence for literally any conduct if the intent was to assist in terrorism. Procuring vehicles, preparing lunch, giving a pedicure.
Lawyer in TFA seems to indicate that the met went on some kind of fishing expedition in the accused's browsing history, but a read through the linked press release [0] tells a different story :
Count 1: Membership of a proscribed organisation. On or before 22 September 2016 Samata Ullah belonged or professed to belong to a proscribed organisation namely ISIS (Daesh). Contrary to section 11 Terrorism Act 2000.
Count 2: Terrorist Training. Between 31 December 2015 and 22 September 2016 Samata Ullah provided instruction or training in the use of encryption programmes, and at the time he provided the instruction or training, he knew that a person receiving it intended to use the skills in which he is being instructed or trained for or in connection with the commission or preparation of acts of terrorism or for assisting the commission or preparation by others of such acts.
Count 4: Directing terrorism. Between 1 December 2015 and 22 September 2016 Samata Ullah directed the activities of an organisation which is concerned in the commission of acts of terrorism. Contrary to section 56 Terrorism Act 2000.
I would have thought those - if sufficiently evidenced - would be enough to establish intent [2], which is what kicks in Section 5.
So he wasn't charged 'because encryption' he was charged 'because terrorism'.
Please note that I'm not making (here) any argument for or against S5, just pointing out that the reporting of this case is misleading as to the legal reasoning.
[0] http://news.met.police.uk/news/man-charged-with-terror-offen...
[1] http://www.legislation.gov.uk/ukpga/2006/11/section/5
[2] https://en.m.wikipedia.org/wiki/Intention_in_English_law
Edit: formatting