I had some scally trying to get through my window at 2am the other morning. Luckily - for him - when I pulled the curtains back and asked him, "What the

he thought he was doing" he scarpered. Had he actually gotten in I wouldn't have been weighing up what would be considered "reasonable force" to remove him.
The law should surely realize that when somebody is confronted with an intruder they may not be thinking rationally. The adrenalins pumping, and all you can think of is that you're in danger and must protect yourself.
If he had gotten in I would have used whatever force came to me at the time.