The London borough boundaries with fewest crossings

Some London borough boundaries are easy to cross. The boundary between Croydon and Bromley, for example, is 10 miles long and can be crossed in over 40 different places. But other London boundaries are much harder to cross, or in a few cases impossible except by boat. So let's investigate... The London borough boundaries with fewest crossings It's best to do this in two parts. a) Boundaries along the River Thames Starting out east... Havering → Bexley: 0 (big estuary, no crossing points) Barking & Dagenham → Bexley: 0 (ditto) Barking & Dagenham → Greenwich: 0 (technically 1 if you catch a Thames Clipper from Barking Riverside to Greenwich, but physically zero) Newham → Greenwich: 7 (three railways, one road tunnel, one foot tunnel, one Dangleway and arguably the Woolwich Ferry) Tower Hamlets → Greenwich: 5 (two railways, two bores of the Blackwall Tunnel, one foot tunnel) Tower Hamlets → Lewisham: 0 (boundary is less than 1km long, so zilch) Tower Hamlets → Southwark: 4 (one bridge,…