Should I ride my first fixie back from the shop (in London)?

Oh really? Oh well, I'll need to go there anyway to find one so maybe I won't be picking up Tuesday ...

I'm not very good at this.

Don't worry mate. Nobody is when they first get into anything. Both 14BC and BLB are OK to talk to as long as they don't think you're a tyre kicker. They answer any questions and give you plenty of advice. Half the fun is looking at all the gorgeous bikes. Enjoy yourself.