Bad luck Flube
I suppose its a silly question to ask where you stand with your travel insurance?
You did have travel insurance didn't you?
If so, loss of documents and the following consequence should be covered if you have a decent insurance.
I'd take gary09's advise, look at your online options for best price single ticket, make a note of it, then visit a travel agent (sorry, not familiar with BKK travel agents to help) and see if they can better or equal it.
Edit- Maybe this link can help with a cheaper fare back to London? 17,800baht +taxes one way
Flights from Bangkok to Europe - travel.thaivisa.com