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

Okay cheers guys, I'll prob be at BLB/14 Bikes at around 9am as I have other stuff to do in the middle of the day, so I would think the deal should be sealed by 11 at the latest if anyone wants to come and mock/sherpa me.

I thought you were collecting a pre-ordered bike. You'll be lucky to find exactly what you want in the right size out of stock. Usually you'd need to do the deal, then come back to collect when the bike's been built and set up.