Dum-dum bullets and shoot to kill
The Metropolitan Police are armed with bullets that the army wouldn't be allowed to use.
Hollow-tipped bullets are designed to expand inside the body on impact, making them much more likely to kill whoever they hit. The generals of the 19th century decided that they were too lethal for war, despite British arguments that they were needed to stop 'fanatical barbarian[s]'.
So they were banned under the 1899 Hague Convention**, along with chemical weapons and bombs thrown from airships. NATO still keeps to that, though perhaps few other armies do.
The police, through, are unaffected. So the Met used these these bullets to kill Jean Charles de Menezes, and this year they started using them more generally.
Now, there's a decent argument that this isn't as bad as it sounds. If a bullet is busy mashing your victim's organs, it's less likely to pass through and hit whoever is behind them. I thoroughly approve of not killing bystanders, but a little confused how that works alongside the policy of giving machine guns to the transport police
Or, more fundamentally: the UK police already kill about 90 people per year -- let's not make it any easier?
[disclaimer: I know nothing about guns, would be happy to never see one again in my life, and wish the police agreed]