I do a variation of most of the above routes. It's pretty safe most of the way on the backroads. I use Blackfriars Bridge. The bridge itself is fine but you need to be very careful up to Ludgate Circus. I've tried Southwark Bridge but I think the traffic is worse once you get to the other side. I work in Farringdon (near Holborn). I'd be happy to ride with you one morning. Some mornings I leave at 8.20am or 9am if I do school drop off. It takes about 40 minutes at a leisurely pace.