I transit the City zone via the Dome on a regular basis and it seems to be a fairly well used route. From the south I usually route via tonbridge to stay clear of Biggin and it seems to be fairly uncluttered on the ground. To the north, normal routing is up the Lea Valley lakes which gives options (although wet ones) if the donkey quits. It's a great route to fly on a clear night with Canary Warf sparkling away. Heathrow Radar are good for SVFR clearances through the East of their zone via Burnham and Ascot as well.
-S