Story time! Sorry for the wall of text.
A couple of years ago I stepped in to help a security guard from Tesco who got into a scuffle out on the street. The scumbag hoo had stolen a bottle of vodka pulled a knife and let me tell you, there was a very immediate sense of regret where I went from trying to restrain the guy to trying convince the security guard to back off and just let the guy go.
The guy didn't look that big but I guess adrenaline/drugs were involved because he was easily overpowering the two of us. There was an audience of maybe 30 people gathered round but no one else stepped in. We managed to get him on the ground but the security guard took a nasty cut to the head and was hospitalised, I, amazingly (having never been in a physical fight in my life, though have done restraint training as I volunteer with teenagers), got away with just a graved elbow. However, my girlfriend who was with me and watched it all was in tears for a good 30 minutes and was quite upset with me. By this point other security guards from nearby shops had come out and the police were arriving.
On the topic of proportional self defence, I got a good kick in the guys ribs when he was on the floor, but he was still flaying around and trying to get his knife hand up in the air. In that situation I think anything I did would have been classed as self defence as the guy had a weapon and there were lots of members of public/kids etc nearby.
I personally think in these situations you are unlikely to be found guilty of an offence by a jury of your peers if the person is attacking back, they are posing a risk to yourself and others. I would be more concerned with personal safety than legal repercussions.
I'd like to think I'd do the same thing again but it is an experience that has stayed with me so it's hard to say until push comes to shove. The blade was tiny so unless he went for the jugular or a major artery, I don't think it would do too much damage, but then who's to say a lucky shot or the next guy wont have a bigger knife (insert "that's not a knife, that's a spoon" joke).