This was so long ago and not clear cut as the guy in England and him were interacting, some of the same people working on both campaigns. He is former VP and starts there.
Other allegations of past law school plagiarism and exaggerating his academic record soon followed. Biden withdrew from the race later that month.
https://en.wikipedia.org/wiki/Joe_Biden_1988_presidential_campaign
Biden plagiarized a speech by the labor leader in England, Neil Kinnock. If slow Joe had only given credit to Neil Kinnock it would have been not an issue. Biden came down with the stupid that day. This happens often with “slow Joe.”