The line your deadline falls on
Most corridors have one route. This one has two, separated by a date. Document legalisation for Thailand runs through a consular desk today, and from 28 February 2027 a single certificate takes over that work. Nothing about your papers decides which applies to you; only the day the file has to be ready does.
Where Thailand stands
Thailand has joined the Hague Apostille Convention, with effect from 28 February 2027. Before that day the shorter arrangement is simply not operating, so an apostille issued in Reading for use in Thailand is not yet the answer. After it, the same certificate is expected to do the whole job on its own.
What the two routes share
The origin half is identical under both. A certified copy is obtained where the record is a civil one, and The Legalisation Office certifies the British signature or seal on it. An apostille from the United Kingdom to Thailand and the longer consular version both start there, which is why the early work is safe to do now.
Counting the charge and the calendar
The British stage is charged per document, at £45 for the standard postal service, and it works by post rather than at a counter. Add the posting time in both directions to whichever route applies to you, then compare that total against your deadline. That comparison, not the treaty, is what should settle your decision.
Writing the plan down
What you need for an apostille for Thailand today is patience and a date; what you need for a file going now is the full consular route. Write the deadline at the top of your list, mark 28 February 2027 beside it, and let that pairing choose the plan before you spend anything.