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

Can't take Circle line any time, whether peak hours or not. It's one-a those underground lines.

Crap, can't find the link for the one I'm looking for but I just asked about it, so do a search for TfL bicycle tube map or something.

you can, any sub-surface line allowed bike, it's only when it's rush hour you're not allowed completely.