FFF,
If you get a clearence not above 2400ft from Thames Radar to cross the Dome and then up the Lea Valley I have always worked on the assumption that if the engine quits then I would use LCY (EGLC) in my emergency plan.
Approx. 9:1 glide ratio gives max range of about 3 to 3.5nm in still air roughly.
Is this not such a good idea.....Not a challenge to you, just want to check this idea out to see if you or others think this is sailing close to the wind?