I am also tempted by (i) the X80 Chafford Hundred to Greenhythe bus - which would have to be in that direction, as this is the only bus that uses the QE2 bridge. However clearly including buses sets a dangerous precedent. That way lies madness etc.
I guess it could be
Aylesbury Vale Parkway to Marylebone
Marylebone To Baker Street
Baker Street To St Pancras International
St Pancras Internation To Gravesend
Gravesend To Tilbury (ferry)
Tilbury Riverside To Tilbury Station (allegedly free bus)
Tilbury To Chafford Hundred
Chafford Hundred To Greenhithe (X80 bus, across QE2 Bridge)
Greenhithe to Woolwich Arsenal
The Woolwich Tango involving the ferry, the foot tunnel and the DLR and (if at the right time of the day the Thames Clipper)