While in the US there is a generally accepted right to self defense, the legal theory in the UK is that fighting crime is the police's job.
This brings up a question. It's well established in the US that the police have no responsibility to protect your life; if you call 911 when the Bad Guys show up and get killed before the police arrive, your next of kin don't get to sue the cops. (Look up Warren v. District of Columbia for an example.) It's not that much of a problem here as you have the right to defend yourself, with deadly force if necessary. In a legal environment where that right to self-defense isn't guaranteed, as it isn't in the UK, does that then imply a potential liability if their police don't do what has been decided is their job? (I suspect it doesn't, but I could be wrong.)