If you ignore the 80 years of oppression that preceded the Hamas attacks on Israel last year, the Israeli response has been one of genocidal intent. From indiscriminate bombings to cutting off supplies of food, water and other aid. They have directly killed at least 40k people, and likely many more from starvation and preventable diseases.
This could have been easily avoided by a simple prisoner swap. Israel has thousands of Palestinians detained without charge, and Hamas wanted to free them.
From what I understand, it seems like him saying how Israel hasn’t and cannot defeat Hamas and Hezbollah has been interpreted as being support for both groups, which are designated as terrorist organisations by the UK. That’s the grounds for the arrest.