That probably woldn't work. The van needs to get to Dunwich in the first place.
Hire van, put own bike in van, drive it to Dunwich on Saturday morning/afternoon.
Get bike out of van, cycle to Darsham station.
Get train to London (not a problem before DD).
Get to Hackney Fields for start of DD.
Ride DD.
Arrive at Dunwich Sunday morning.
Load van.
Drive back to London.
???
Profit.
Bonus points for leaving towels, beers, gas stoves, sausages, etc in the van for your arrival.
Hire van, put own bike in van, drive it to Dunwich on Saturday morning/afternoon.
Get bike out of van, cycle to Darsham station.
Get train to London (not a problem before DD).
Get to Hackney Fields for start of DD.
Ride DD.
Arrive at Dunwich Sunday morning.
Load van.
Drive back to London.
???
Profit.
Bonus points for leaving towels, beers, gas stoves, sausages, etc in the van for your arrival.