Starts well: “Everyone has the right to freedom of expression. This right shall include freedom to hold opinions and to receive and impart information and ideas without interference by public authority and regardless of frontiers.”
But then: “The exercise of these freedoms, since it carries with it duties and responsibilities, may be subject to …” (lots of conditions!)
Communications Act (Malicious Communications) is the law he broke, it covers sending "offensive" messages online.
Policing and Crime Act (PACE) is where you need to look for arrest/interview/search procedures. Very different to the US.
Hate symbols usually come under Mal Comms (online), or Public Order.
There is no constitutional (or any, really) guarantee of free speech in the UK.