TLDR : Answer Summary
When flying with EVA Air from London to Bangkok, it's important to be aware of the onward travel requirements. While some travelers report not being asked for proof of onward travel, it's formally required according to Thai regulations. Generally, a bus ticket (e.g., Nong Khai to Vientiane) may suffice, as some travelers have used this successfully, though experiences vary. It's advisable to have a ticket for your next destination ready, either a flight or bus, especially since some airlines might not allow boarding without an onward ticket. Overall, while many have traveled without issue, having proof of onward travel can provide added security.