The problem here is that contextually you are falling into the trap of "talking about committing a terrorist act" as being relevant to "having private communications", and in the process you are conflating the two. This means you are falling into the trap that the UK government intentionally creates to suppress privacy — within a reader's head, now the two are related. This also means you haven't had to develop any arguments other than "muh free speech!" with respect to why having private communication is important.

The second problem is that American conservatives have framed Nazi speech as a free speech issue, so to an onlooker who is not in the USA, when people talk about "free speech", it comes across as someone defending someone's right to say incredibly harmful, violent things about Jewish people, Transgender people, and so on. I think for most people outside of the USA (and, to be honest, most minority populations within the USA) you should consider "free speech" as being an incredibly tainted phrase for that purpose.

The flipside of all of this is that fascism is very, very possible even with freedom of speech (actually it seems to rely on it, given how virulent the spread of outright Nazi rhetoric has been in the USA so far). Freedom of speech is not the sole thing that holds up a democracy and it weakens your arguments for you to rely upon it like this.