It still applies.
But the right of self defence in British law only justifies the use of deadly force in cases where a person has reason to belief they or someone else is in danger of death or serious injury. You don’t automatically have the right if your property is threatened (but you do if you are also in danger and a lot of the time you will be).
Everyone seems to have heard of the Tony Martin case where he was convicted for killing a burglar - but he was convicted because the burglar was running away at the time, and was no longer a threat - and Martin admitted he was no longer a threat.
You don’t hear anywhere near as often about the numerous cases where a person was found to have acted legally in killing an intruder (such as the Tony Evans case), because most of these are only in the newspaper for a couple of days before it’s clear there will be no legal action - they don’t drag on for weeks in the courts gathering publicity.
In this day and age, "running away" doesn't automatically mean that there is no longer a threat. Gone are the days where you can say about someone running away with their tail tucked between their legs "I taught him a lesson", and then you go on about your business.
Many times, and in the more violent places, most of the time, the criminal "runs away" to come back again with more "fellas", more deadly weapons, or what have you.
Everyone seems to have heard of the Tony Martin case where he was convicted for killing a burglar - but he was convicted because the burglar was running away at the time, and was no longer a threat - and Martin admitted he was no longer a threat.