I live in USA and have done for many years, I am quite aware of the gun threat thank you very much.No. The officer said 'I think it's a cap gun'. He didn't say 'It's a cap gun'. There is a huge difference.
How long should he have investigated for? It's not the response officers role to investigate. He is there for an immediate response. It's for others to investigate after things have calmed down and the danger has passed.
Again, don't judge this by the relatively low threat of gun crime in the UK. The gun threat across the pond is very different.
Yes, the officer is there for an immediate response but that immediate response should not be to fire without determining a) that there is an actual threat b) that if there is a threat it can only be resolved by firing rather than de-escalation techniques c) by firing they are not putting other lives in danger. I don't think this officer did any of them.