It in indeed terrible. It's partly due to borough boundaries and different boroughs having different default speeds. The particular section that annoys me the most though for this is the A107 which runs from Seven Sisters Road, through Stamford Hill, Clapton and to Lea Bridge Roundabout. Lea Bridge Road is ok but the speed does change at the Waltham Forest roundabout. There's also multiple bus lanes which have different hours of operation and generally it's a bit of a mess.

I feel obliged to say I only do this in my car when I'm providing assistance to a mobility impaired friend!!! Usually I'd be on the bike and in the bus lane.