Grand Theft Auto: London 1961 has no in-game completion percentage, no reward for doing everything and no achievements. It is a free PC-only add-on that needs Grand Theft Auto and London 1969 (PC) installed. Progress is the chapter's target score, and the game counts its Kill Frenzies as "secrets". This checklist follows the community 100% definition (gta.wiki) for the single-player chapter When We Were Very Small: reach 100,000 points, complete all 7 missions (4 phoneboxes, 1 walkie-talkie job, 2 vehicle missions) and all 7 Kill Frenzies — 15 objectives in all.
The opening introduction and the ending meeting are not listed separately (the ending unlocks with the target score). Manchester is a multiplayer-only deathmatch map and is not part of this checklist.