As of today, the easiest route onto a Destination Thailand Visa has closed.
Two changes take effect on 31 August. The first is the one that will catch most people out: you now have to be a national or a permanent resident of the country where you submit your DTV application. Being physically present is no longer enough. The Royal Thai Consulate-General in Savannakhet now states plainly that applicants must be Lao nationals or permanent residents of Laos. The Royal Thai Embassy in London has published the same logic in reverse, making proof of permanent residence in the UK, Ireland or the British Overseas Territories compulsory.
If your plan was a cheap flight to Vientiane and a DTV in five working days, that plan no longer exists.
The second change is a criminal record clearance certificate, issued by the authority in your country of citizenship or legal residence. London is asking for a police certificate issued within the previous six months. That is not a document you can produce in a week from a guesthouse in Laos, which is the practical part people are underestimating.
Applications with the fee fully paid before today are still assessed under the old rules. Everyone else starts from the new checklist.
This is something the TDY community deals with constantly, and the honest read is that Thailand is not closing the door, it is filtering who walks through it. Plan your application around a trip home rather than a border hop, and start the police check early.
One caveat worth taking seriously: the exact documents, translation and legalisation requirements vary by embassy. Check the checklist of the specific mission handling your application, and speak to a licensed visa professional before you book anything.