Here's the latest on the heckler...
https://www.bbc.co.uk/news/uk-scotland-edinburgh-east-fife-62889396
Interestingly this isn't a criminal offence in England and Wales, only in Scotland. However the ECHR requires that citizens know what actions will break the law - it may be that a good defence barrister will argue that the arrest is contrary to this chaps human right to protest. If there is no evidence of actual "alarm" then the conduct must be "flagrant" - a standard of conduct which would be alarming or seriously disturbing to any reasonable person in the circumstances.
Personally I think a better approach might have been to have a quiet word. "Look - I appreciate you want to protest, but this blokes just lost his mother. Have a think about what you'd like to happen at your mum's funeral. Don't do it again."