You are now in the bonus level. This is apparently what happens between Thames Wharf and Aldwych.

Climb onto the block in front of you. And from there up onto the metal grating. Take a running jump onto the next metal walkway. Turn right and make another running jump and grab the ledge of the next grating. Turn right again and make another running jump. Climb up. On your right is a dangling platform. Climb onto it.

Goodies:
Turn left and jump onto the platform there. Jump straight up to get to the platform above. Collect the uzi ammo. Climb back down onto the previous platform and drop further down onto the platform on the right. Step onto the sloped part. Here you have to jump up to the ceiling and grab hold. Shimmy along and over the railing onto the brown pillar. Let go. Here a flock of bats will fly past. Jump towards the gap in the wall and hold on to it. Pull in and collect the small medipack. Climb down into the pit on the other side. Here you will find a crystal. Climb back out and drop back onto the ground. Climb up again till you reach the dangling platform.

Continue with a running jump towards the platform in front. Grab the ledge and pull up. Collect the flares. Turn around and look towards the pillar left of the platform where you came from. (Picture.) Take a running jump to get there. Don't grab the ledge! Jump up and hold on to the ceiling. Climb forwards. Then drop down onto the sloped block. Slide down and grab the ledge. Make sure that your energy is full. Then let go and drop through the floor.

Collect the large medipack. Turn towards the platform on the left where you can see flares and take a running jump to get there. Collect the flares. Further to the left you can see more platforms below. Take a standing jump down there. Drop into the pit to collect the large medipack and the uzi ammo. Climb out again. Jump onto the black and white walkway. Climb up into the passage and pull the lever to open a trapdoor elsewhere. Climb back outside. Step towards the gap in the railing and drop onto the platform below. Take a running jump to the platform on the other side and grab the ledge.