Lala - you've got no chance of knowing why Harry left unless you've dedicated your life to flipping back and forth between AmEx and TA.
Basically, Fallon, Harry, Jazzer and Kirsty drove Fallon's van up north, trying to get to the Edinburgh festival, which was really supposed to be Fallon's chance to finally get off with Harry (Jazzer had invited himself along and Kirsty was invited to try to distract Jazzer). They broke down on the remote Scottish borders, but luckily Harry recognised it as being only a few miles from where the great love of his life lived. The love of his life turned out to be a young man called Grant, who has an adorable dog, a wood-burning stove, a very nice house, and his own iron, breadmaker, food processor, etc etc. Sadly they had quarrelled, leaving Harry to throw it all up and become a milkman in Ambridge and share a flat with Jazzer. On remembering what he'd been missing, Harry realised that he would better getting back together with Grant, leaving Ambridge one milkman short of a float.
Thus, the joke is that after all Jazzer's suspicions of a tidy cyclist who loves to cook, Harry was gay all along.
The scene with Harry giving Jazzer his stuff was definitely on TA, but it wasn't a very exciting episode.