Rewards: Pounds: 2490 (+125), EXP: 3500 (+150), Extra: Outfits - Master Assassin
Challenges: 1. Find the secret passage.
When facing the Bank of England, climb down the building you're currently standing on (or perform a leap of faith) and approach the bank from the left side; climb up and check out the nearby roof. One of the windows on the roof is open (that is, one of the windows you can stand on as part of the roof's platform), allowing you to drop down on a chandelier. First identify the head of security with the right stick. As he moves into this room, he'll stand towards a wall for a short while. This is an excellent time to drop down, approach him from the back and tap the Circle / B button to kidnap him. (Be sure to have identified him first, though.) He'll tell you Twopenny's weak spot, which is all the info you need from him, so knock him out by strangling him.
Ignore the adjacent room and use the rope launcher to reach the window you came through. From the roof, continue around the northeast corner and keep walking until you reach a lower entry point/open window (not the first, higher one). Hop inside the window and wait for the bank manager to pay this room a visit. When he turns his back on you, immediately drop down and kidnap him. Sometimes one of the guards in the next room becomes suspicious and checks out the situation, but when this happens all you need to do is quietly walk away from him towards the desk until he moves away. (And in case you were wondering, we'll get to the chest in this room at a later time.) Proceed through the next room and completely ignore the vault watcher to the side. Move into the vault to the right - the guards will step aside for the bank manager you kidnapped, but be sure to walk very slowly to reduce the range of the suspicion circle. Once past the guards, knock out the manager in the next room.
Head down the stairs and scan the area when you reach the next room. There are a ridiculous amount of Blighters/Templars in here, so it's advisable to take a stealthy approach. One solid way to go about things goes as follows. Head left and move to the end of the walkway and hide behind the cabinet with gold bars. A guard will soon approach this part and look towards the wall; when he does so, kidnap him and slowly walk downstairs. On this floor, various enemies are busy moving gold from one position to another. Stay out of their range and move towards the pillars on the left side, then through the doorway and enter the room straight ahead (slightly to the right), which is where the unique kill opportunity is located (as indicated by a skull marker, if all went well so far). You can safely strangle the guard here in the right corner, out of sight from the others. Hide behind the painting and assassinate the capitalist b**** Twopenny by stabbing him in his throat. (It really makes you wonder whether games sublimate popular, reasonable anger against the top 1% wealthiest.)
Although you could technically just follow the waypoint, you shouldn't do this if you want to find the secret exit passage. You'll first have to kill the guard standing right in front of the doorway up ahead. You can do this with a throwing knife headshot to be 100% safe, but luring him is probably possible as well. Wait for the patrolling guard to have his back towards you, then rush through the doorway to the far end of the room, staying near the wall, and killing the enemy standing with his back towards you. Continue around the corner and make your way to the end of the hall, then turn left and head outside. Slide underneath the hole to reach the secret passageway. Make your way to the end of it and climb up.
In this room, defeat the guard in any fashion you like, then use Eagle Vision to spot a chest behind some stuff in the corner. After collecting its contents, open the locked door on the other side and you'll be back in the room where you kidnapped the bank manager, allowing you to open the chest here and escape via the window. Simply exit the premises to finish this memory.