I came out at the Huai Xai / Chiang Kong border.
6 months UK and back is very tight and has lots of hurdles in terms of Visas, or do you plan to Ship it from somewhere on the way back (Vladivostok would be good but make sure you aren't up there in winter!). Do not ship back into Thailand if you overstayed your temporary export my shipping agent warned me, she said it would be much better to ship to another country (cambodia/malaysia) and take a land border.
Problems at a land border can be sorted one way or another, at (air)ports you pay for every day you can't get the motorcycle released, often > 100$ a day.
Carnet for a Thai bike can be obtained from foreign issuing authorities, my favorite is Switzerland, fast and extremely friendly.