When are you coming over? time and circumstance may provide you with willing guides to either city.
Sorry can't give you exact shops - I suppose it depend on what you want in a steampunk shop
In London you Might want to head to Camden Stables Market. As well as the expected and acclaimed Gothic stores there are ex army stores, fashion stores, bric a brac and antique merchandise.
http://www.stablesmarket.com/ If you're into Gothic influenced steampunk (by that I mean more Sisters Of mercy type Gothic, not Arched Vaults and flying buttresses ) Camden in general would be a a good place to go.
You could also go to Brick Lane Market - a bit like a cross between a street market, a yard sale and a den of thieves. Off Brick Lane there are a couple of buildings that have their own markets inside
http://www.backyardmarket.co.uk/cgi-bin/web/index.plhttp://www.sundayupmarket.co.uk/index.htmlCome to think of it, there's alot of buildings that have their own little markets - they just don't have web sites. Alot of the warehouses around Brick Lane turn their space over to market traders on a Sunday so there is a whole lot of variation going on in the market. There's also a "car Boot" feel with traders just setting up pitch along the road, or any of the side roads. It does have a reputation for second hand bikes - actually it has several reputations for second hand bikes, however if I repeated them I could be done for spreading malicious rumors

- after all there is only a small percentage of stalls are selling bikes - kosher or otherwise.
Within easy walking distance of Brick Lane market is Spitalfields.
http://www.visitspitalfields.com/Petticoat Lane Market is also close by, know for its fashion. Although not steampunk I have gotten some nice shirts from a vendor who goes there.
Portobello road is known for its antique dealers
http://www.portobelloroad.co.uk/ For a drink I might recommend the Princess Louise public house in High Holborn, if only for the gents lavatory (which is listed - see picture number 4).
http://www.beerintheevening.com/pubs/s/19/194/Princess_Louise/HolbornThere's also The Olde Mitre Tavern, which is sooo easy to
not find. I've been there loads of times but still wander around aimlessly at times trying to find it - but once you HAVE found it it's such a delight.
http://www.pubs.com/main_site/pub_details.php?pub_id=165%22Then there's The Jerusalem Tavern - small and quaint but a great little find
http://www.stpetersbrewery.co.uk/london/default.htmYou know, that seems to sum it up for me, I start off shopping but end up in the pub
