The 121 was the latest finisher at 0221, until the short "154N" journeys were introduced. I ran a Pointless style quiz a question along these lines a year or two ago, will paste in the answers I came up with, though some might be slightly out now due to schedule changes.
Q. Name a day bus route that is scheduled to be in service for any length of time between 01:30 and 03:30 inclusive on at least one day of the week. The route must not be a 24 hour route or have an N-prefixed night element.
A. Originally considered using 02:00 as the start time, but changed it to 01:30 to make it a bit easier. Should have added a bonus to routes given still running after 02:00 instead. Interesting that most answers given were routes that finish late, rather than those which start early. It's mainly the airport routes that start really early (most serving Heathrow Central are 24 hour.) Late finishing routes tend to be those running through many areas with no night service, or routes where the garage is close to the last stop. The R68 has a few garage trips that run in service up until almost 02:00.
Some day routes running at or after 02:00; 75, 121, 154, 157, 177, 307, 358, 493
Some day routes running at or before 03:30; 350, 358, 423, 482, 490
The 106 and 422 also start running before 04:00. The 358 route is the only day route I know of that satisfies both criteria.