Today's route, and the date that changes it
Start with the date, because it decides which job you are doing. An apostille for Thailand is not accepted today, so a file going there is closed by embassy or consular legalisation. From 28 February 2027 an apostille takes over that step. Both halves are true at once, and only the second of them carries a date.
So an apostille from United Kingdom to Thailand is not yet the route, whatever a form or a website suggests. Document legalisation for Thailand runs through a consular desk until that day arrives, which means a paper wanted before then follows the longer path from beginning to end.
The one choice the British stage gives you
Most of this route is fixed for you. The single place where you genuinely choose is at The Legalisation Office, which takes work by post at £45 / document (standard postal); £40 / document next-day (registered businesses). Two prices, two speeds, and the quicker of them costs less rather than more.
The catch is who may use it. The next-day service is for registered businesses, so a person sending a family paper is on the standard postal route by default. That is not a penalty, only the shape of the service, and it is worth knowing before a timetable gets built on a speed you cannot buy.
What the faster setting does and does not do
A quicker British stage shortens one leg of a longer chain. On a route that still finishes at a consular desk, the days saved sit in the middle of the job rather than at the end, and the final date depends on stages nobody in Britain runs. Buy the speed for a reason you can name, not for comfort.
There are good reasons. A business moving several papers at once gets them back in a rhythm it can plan around. A near deadline can turn a handful of days into the difference between making a date and missing it. Outside those cases, standard postal work performs exactly the same official act at its own pace.
The paper still comes first
None of the speed helps if the wrong sheet travels. For births, marriages and deaths, Birmingham Register Office at Holliday Street, Birmingham B1 1TJ issues certified copies, so a fresh copy can make the journey while the record you keep at home stays exactly where it is. That order of work never changes.
Numbers matter as much as speed here. An apostille issued in Birmingham for use in Thailand becomes the finished item on 28 February 2027, and until then every document travels separately through both stages. Ordering all the copies you need at the outset is the cheapest decision on this page, whichever price you pay at the British desk.