Is a Nong Khai to Vientiane bus ticket valid proof of onward travel from London?

Yes - we (UK) did this nearly a year ago and had to present the booking at BKK immigration. I also booked refundable accommodation in Vientiane to back that booking up. It was accepted.

i think it depends on airline some need a flight out the country

Did it myself no problem

if it has a date on it, it should be satisfactory as POOT. Some officials may not be aware of this

No, Nong Khai is a Vietnamese autonomous - Are you freakin' happy now?

In general yes.

Yrs border to lows on the river

Yes. A confirmed bus ticket from Nong Khai, Thailand to Vientiane, Laos can count as onward-travel proof. The Royal Thai Embassy in London specifically says visa-exempt travellers can show a confirmed air, train, bus, or boat ticket leaving Thailand within 60 days.

So if you’re flying London → Thailand and then taking the bus to Laos, that ticket should generally satisfy the onward-travel requirement. Make sure the ticket is confirmed and clearly shows your name/date.

If you’re applying for a tourist visa from London, check the visa category too, as the embassy may ask for the complete travel itinerary.

You could consider reading a book or two before your trip. I’d recommend Unlock Thailand by Isla Rey on Amazon—it includes all the info and insights you need. You should check it out. A very good companion

Yes but make sure you get your Laos visa online. The bus will not wait for you while you process through immigration.

Google says no lol..but bus train or plane.ok

Yes

Yes before living here that's the system I used.

Book online with PDF with ticket confirmation

Yes

I used a train ticket from Bangkok to Laos