You will need to meet the cost of this flight yourself. You do not need to pay before you travel. You will have to sign a commitment to pay before you board. The cost of the flight will be based on a normal price for a ticket and charged at a later stage after the flight.
You must also organise your own flight from Zurich to UK. You do not need to do this before you travel but you must remain in transit in Zurich until you organise one. There are flights available to London.
More information on the Embassy of Switzerland website: www.eda.admin.ch/bangkok
Advice from the Swiss Embassy is that if you are unable to register you can also show up at the counter before 8pm.