England has to pay the money only if England wants to use the price as an excuse to not exit after all. Otherwise EU can go whistle for their money.
I guess they will just load up pallets and send them over.
Issue is market access. Post the money in escrow, then hammer the EU on the remaining points. Good negotiators will keep the 20 billion pound figure in mind and negotiate accordingly. The UK can come out way ahead on this one.