Kovacs is correct re "reasonable force". In the UK, you do not have the Right to kill or even seriously injure a burglar. You must genuinely believe that your Life is in Mortal danger if your defence is to Accepted by a Court. Otherwise, it's just your word against his and this will be tested in court.
Re Armed Police. Our Police are unarmed as a matter of course. Armed Police are now more visible in specific areas (Airports - Whitehall) as a visible deterrent in the first instance. Police also use Armed Response Vehicles though these are likely to be relatively few in number.