Time for some context from a Brit who's now an American....
The US constitution guarantees freedom of speech even if it's offensive to others. In Britain that's not the case and there is no comparable constitution. The Brits have an established precedent of free speech but not if it's inciteful, malicious or offensive to society. British law is a little vague about what language may incite violence or other extreme behaviour but generally speaking policing authorities only get involved with cases such as neo-nazis or, in more recent years, occasional Muslim fanatics inciting terrorism.
In the UK, the poppy is worn in November every year in remembrance of deceased military and originally came from the poppies that bloomed on the battlefields of WWI. Burning poppies at this time of year would be as offensive to many Brits as burning the US flag in July would be to many Americans. The kid concerned was not handcuffed, that generally only happens in the UK with someone who is being violent, but he was arrested and held for 24 hours then released on bail. It's a grey area of the law and it apparently hasn't been decided yet whether he will face charges.
Here in the US we have almost unlimited freedom to offend others, in the UK there is a threshold but it's not defined.