Where Thailand stands today
Say both halves of this in the right order and nothing goes wrong. Today an apostille for Thailand is not accepted, and papers meant for use there need consular legalisation through Thailand's mission in the United Kingdom. From 28 February 2027 an apostille replaces that step. Both statements are true, they simply belong to different days, and the whole of document legalisation for Thailand turns on which of those days your file lands in.
The date that decides is the presentation date
People reach for the wrong date, and it is always a date they can see. The issue date printed on a certificate feels official, so it gets used. The day you first walked into the register office feels like a start, so it gets used too. Neither has any bearing here. The rule attaches to the moment a completed file is handed to the body that will read it, which is usually the last date in the whole story.
That has a practical edge to it. A file you begin now and hand over well before the switch belongs to the consular chain, however close the change may look, and a file begun now but presented after it does not. So an apostille from United Kingdom to Thailand is not something you can bring forward by starting early; starting early only buys you room inside whichever route the presentation date puts you in.
The part that does not change
Everything on the British side runs the same way on both routes. Sunderland Registration Service issues certified copies of births, marriages and deaths, so a fresh copy travels and the certificate at home stays put. The FCDO Legalisation Office compares the signature on that copy with the specimen it keeps and attaches its certificate, working by post with no counter to visit. The routes differ only in whether a further stamp is needed afterwards.
Planning around a moving deadline
Where a deadline sits near the changeover, plan for the slower chain. The consular stage adds a desk, a charge and a postal leg, and those weeks are hard to recover once lost. An apostille issued in Sunderland for use in Thailand is the endpoint of the shorter route only once the change has taken effect there. Preparing for the longer version costs a little time; guessing wrong in the other direction costs the whole run.
Cost, and why early work keeps its value
The reassuring part is that the two routes share their first and most expensive habits. The £45 standard charge per document and the £40 next-day tier for registered businesses apply to the British stage regardless. A certified copy ordered today is just as good in either world. So the sensible move is to get the British half finished and hold the result, then let the presentation date decide whether one more stamp is still needed.