I love RPGs. I used to play at Uni. I miss it, but I know no one out here who plays...
So, here are a few ideas for steampunk RPGs set in London.
1) Joseph Bazalgette needs the heroes help. It is 1860 and he is building the sewers, but something dark and dangerous is using the new sewer tunnels as a lair. The heroes must hunt it out.
2) There were 4 assassination attempts on Queen Victoria in her lifetime. It is not hard to make up a fifth attempt, and have the heroes pursue the would-be assassin through Victorian London.
3) It is 1851, the Great Exhibition is in full swing, but suddenly one of the main attractions begins killing people. It is a one hundred feet tall steam-powered man and was last seen leaving a trail of destruction heading towards the river.
4) It is 1888, and Jack the Ripper is on the prowl. But he is not a man; he is a demon from the deepest pits of Hell, and only the heroes can send him back where he came from.
5) A man with a strange tale in the pub sparks adventure. His name is Jonathan Harker, and he spins a tale of a blood-sucking fiend from a distant land praying on the women of our fine city.
6) A mad scientist hires the heroes to bring him dead bodies. Dig them up or kill them yourself, he doesn't care. But when the heroes learn he plans to reanimate the pieces, sewn together as one monstrous man, they must stop him before it is too late...
7) A mad scientist needs a test subject and one of the heroes, strapped for cash and facing debtors jail, volunteers. But when the scientist accidentally makes him permanently invisible, and very slowly becoming permantly intangible, it is up to the heroes to find a cure.
8) The famous Egyptologist Dr Albert Napier returns to London with the results of his recent looting, including a mummy that comes to life each night... think Brendan Fraser's The Mummy in Victorian steampunk London
9) A werewolf in Hyde Park
10) Chinese Triads have a weapon of mass destruction. They plan to test it on Paris, firing from London. This could lead to a massive war between the two empires...
11) Dr Jekyll, Mr Hyde
12) Florence Cook, the famous medium, is haunted by ghosts... indeed, the spirits of the departed are rising all over the city. The heroes must help the restless dead to rest in peace.
13) A sorceror plans to summon Cthulhu to earth... (I love Lovecraft, too).
A nice mix of techie and fantasy ideas, there.
See also http://www.io.com/~sjohn/plots.htm. With the ideas above and this link I'm sure you'll have great games. I'm so jealous...