Just my 2c on attending tech and startup events around London
blog.mailjet.com
blog.mailjet.com
If you want to stay well, and prevent diseases passing between all attendees - scrub those hands!
Developers are no more disgusting than any other human being - so this isn't meant as an insult. But developers in London are an international bunch and are quite often jumping from country to country bringing all sorts of exotic germs with them.
I'd also recommend that you do something with the business cards you get. If you've got Salesforce or similar stick them in there. If not, send a one time email to the people you met reminding them of who you are and what you do. Then ask if they want to go on your developer mailing list.
Don't hand out blank USB sticks. Branded sticks are ok - but unless they're expensive multi-gigabyte sticks, they're useless. Stick your SDK, documents, presentation, demo videos etc on there. When someone asks how they can get started, you hand them a branded stick and you've instantly saved them 30 minutes of digging around your website and downloading files.
Work out which events you need to attend. I once made the mistake of attending a games conference where literally no one cared about our product. A little work in advance will save you that annoyance.
Finally, don't sacrifice your own social life. Most of the events in London happen in the evening and go on late into the night. One of the reasons I left my evangelist job was because I was bored of not seeing my friends and family. It's hard to go out and socialise without feeling like you should be working the room.
Good luck!
Terence
If you're comfortable approaching and talking to strangers, and can talk with confidence about a technical product, that's pretty much all you need to get started.
Once when my voice was a bit worse for wear, I asked a pharmacist for suggestions and he pointed out a commercial product, but said that since the main active ingredient was liquorice extract, I might as well just buy a bag of the stuff at the local store as it would be a fraction of the price.
Fortunately, I like liquorice and I have to say it's always worked for me - although you'll need to consider those nearby if you're walking the room and mitigate the liquorice with perhaps a sugar-free mint.
I don't think that's an either-or!?
(Justin Bieber should not be your role model.)