The above maps are good, but you can unlock the secrets of London using Helix credits so they appear on your map. It costs 1000 credits - but never fear, there is no need to purchase credits with real currency!
During the course of the game you'll unlock plenty of credits for completing some sequences. you'll also unlock some for completed collections such as Helix Glitches, letters from the Front, beer bottles and illustrations (about 100 for each collection). In addition through the ubisoft club app you can get 200 free credits. You can also get 10 for each social helix glitch shared.
I recommend you do this at the end of the game. By the end of the game i had about 1300 credits, so plenty for the secrets map. So save those credits and dont go using them needlessly, as to be fair i dont think much else is even worth it!
just need to go collect them now!