The conference, which took place at York Hall in Bethnal Green, comes after the US press conference in Los Angeles on 16 June.
After walking on stage dressed in a blonde wig, KSI said: “Welcome to London, motherfucker.”
Logan Paul later replied by saying that “it’s good that your entire fanbase is in one room”.
The US YouTuber remained seated whilst KSI continued talking on-stage, making remarks about Logan’s new relationship with the actress Chloe Bennet and his previous experiences in Hollywood.
KSI said: “You’ve been taking Ls for ages. How are you going to try and audition for Baywatch, got the part, [sic] and still not end up in the movie?
“My final song ended up in the final movie. Where were you? Oh yeah, deleted scenes.”
Logan Paul later left his seat without facing off the reigning champion, telling the audience: “See you August 25th.”
Logan Paul got absolutely ruined ahahahaha.. he left the fucking conference early because he couldn’t take anymore hahahahahahahahhahahahahahahahaah pic.twitter.com/SIb7HbCBkA
— Jay Swingler (@JayFromTGF) July 18, 2018
KSI has since commented on the incident on Twitter, tweeting “RAGE QUIT” and an image of the pair with an Xbox connection error message covering Logan’s face.
Jake was met with boos and chants of “Fuck Jake Paul” from the audience, with host The True Geordie asking fans not to throw items on stage.
In a question to Deji, The True Geordie asked at what point he realised that it was him who would take on the American vlogger.
Deji replied: “It all happened in LA, when this bitch refused to fight my brother, so I could step in and fight you myself.”
Meanwhile, Jake was asked whether he picked Deji as an opponent because it was “an easier fight” than KSI.
“I’m gonna fuck up Deji, Logan’s gonna fuck up KSI, then I’m gonna fuck up KSI,” he said.
A few hours later, the official livestream for the fight was made available on YouTube.
The stream will be pay-per-view, with a “July discount” of £6 currently in place.
The news that the stream won’t be free to watch has been met with criticism by fans, with one Twitter user saying having to pay for the livestream “is not an ideal situation for everyone”.
Meanwhile commentary YouTuber Kavos said on Twitter that the decision is “fucking stupid”.
“It’s a YOUTUBERS boxing match and event, how you going to just fuck that over for mainstream media. Disappointed.
“I thought the whole point of the ‘YouTube boxing matches’ was too show the power of YouTube opposed to the mainstream media… such a shame.”
The fights take place at the Manchester Arena on 25 August.
Update: KSI has since responded to criticisms on Twitter about the match being a pay-per-view livestream.
In a series of tweets on Twitter, the YouTuber said: “The cost of the event itself is on another level to last time. Plus the press conferences, weigh-in and making other content for the new channel, all adds up.
“We want to make this as big an experience as possible for everyone watching, without cutting corners. We’re keeping the pay per view price purposely low compared to other events and looking to give as good value as we can.”
He also went on to add that “everything else” released in the build-up to the match will be free to watch.