Even by my incredibly uninformed standards I have little overall idea of freight flows across London beyond there's a load along the NLL/Goblin, a load up the WLL, and some to/from Acton yard. Not to mention the big container ports downstream. It was more tongue in cheek than anything else to set people talking.
So, little more than arbitrarily, Ripple Lane to Wembley/Willesden as a starter for 10? No idea about the available capacity on HS1, but Thamesport traffic could conceivably get to Ripple Lane via HS1 at Ebbsfleet. Thames Gateway traffic can obviously get to RL via the line through Dagenham.
No idea of a good solution for the Felixstowe traffic, though. A branch off the main tunnel towards Romford, perhaps? Or a separate one from Romford-ish to Ripple Lane? Not sure where having additional surface lines starts becoming feasible without looking, but I'm sure Crossrail would like to have the GE electric lines all to themselves.