Mr Merritt's father David said he was a "beautiful spirit who always took the side of the underdog".
"David, I knew your son through Learning Together & I loved him to pieces-he was the sweetest, most caring and selfless individual I've ever met".
Joshua Rozenberg, a BBC journalist who previously trained as a lawyer, interviewed Mr Merritt in February while he was working with Learning Together at a prison in Suffolk.
After studying for a bachelor's degree in law at the University of Manchester and graduating in 2016, the 25-year-old went on to study at the University of Cambridge where he worked in the criminology department running Learning Together.
Khan reportedly threatened to blow up the venue before stabbing several people including Mr Merritt.
As the confrontation moved from inside the hall to the pavement outside, a throng of people could be seen in videos grappling with Khan.
Attacker Khan had been convicted of terrorism offences in 2012 but released from prison on licence in December 2018 and was wearing a tag.
David Merritt, Jack's father, confirmed on Twitter that one of the victims was his son.
"I saw that the knife was still in his hand so I just put a foot in to try and kick him in the head", he said.
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It has also emerged that another of the men who tackled Khan was convicted murderer James Ford, who strangled 21-year-old Amanda Champion to death in July 2003.
Labour Party Jeremy Corbyn criticised the government's sentencing policies.
The Metropolitan Police said the attack is believed to have started inside Fishmongers' Hall at 13:58 GMT on Friday, before continuing onto London Bridge itself, where Khan was shot by armed officers.
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"I think there is also a question about what the probation service were doing. and whether the parole board should have been involved in deciding whether or not he should have been allowed to be released from prison in the first place".
NHS chief executive Simon Stevens said three victims remained in hospital following the attack - two in a stable condition and one with less serious injuries.
"Clearly a key line of enquiry now is to establish how he came to carry out this attack", said Met Police Assistant Commissioner Neil Basu.
Police carried out searches at two addresses in Stafford and Stoke-on-Trent as part of the investigation.