Here is the full list of all 44 Hidden Cats in London achievements. It takes around 0-1 hour to unlock all of the achievements on Windows.
Found 500 cats
Found 480 cats
Found 460 cats
Found 440 cats
Found 420 cats
Found 400 cats
Found All Cats Advanced
Finish with no Hints Advanced
Found 380 cats
Found 360 cats
You found Thunder
Found 340 cats
You found the Wizard Cat
You found Rita Oreo
Found 320 cats
Found 300 cats
You found Darwin
You found Penelope
Found 280 cats
You found Babi
Found 260 cats
You found Lyon
Found 240 cats
Found 220 cats
You found Suki
Found All Cats Normal
Finish with no Hints Normal
Found 200 cats
Found 180 cats
Found 160 cats
Found 140 cats
Found 120 cats
Found 100 cats
Found 80 cats
Found 60 cats
Found 40 cats
Found 20 cats