They should check with their counterparts here as theres clearly been an amendment to the law on the offence of lighting flares/smoke bombs in a public place. Apparently, its acceptable to do it here even in the midst of Police Horses and Mounted Officers though perhaps morons of a special persuasion get a special dispensation.
It's far from acceptable; yet as
@Del-boy mentioned, quite simply with the numbers present vs. numbers of police it's nigh on impossible to act.
If the officers present were to go and arrest one of the flare holders, then it would pull at least two off their actual role of protecting the route.
While I'm not saying it's correct, the more sensible option is reviewing CCTV and hopefully they'll be subsequently dealt with - locked up or banned.
The only other option is to go with much larger number of officers, potentially in riot gear, and drag them out, which will be heavily frowned upon.