Just bring 20,000 bahts worth of Pounds and change it when you get here,much better rates,just make sure the notes are brand new and in pristine condition otherwise they will be rejected at every money exchange and bank.
it'll be destroyed on the terrible roads en route,and if by some miracle you make it to Thailand intact then be prepared to pay 200% of the cars value in import taxes at the Thai border.Its a nice romantic idea...but a totally impractical, hideously expensive,bureaucratic nightmare in practice.Dont do it.