Would there have had to have been a firearm reported, or any other weapon, knife, baseball bat....what is the threshold for armed Police to be sent out nowadays?
nope
I've seen an armed response unit show up to remove a loud mental person from a restaurant in central London - I presume they were simply the nearest unit available at the time and perhaps, as it was a Friday evening, other units were dealing with drunk people etc..
It is also quite standard for some areas to have armed foot patrols too - presumably they are going to occasionally respond to routine incidents too. Given the population of Luton it wouldn't be surprising if armed police were patrolling on foot around there too.