Where Japan stands
Japan joined the Hague Apostille Convention in 1970, and the corridor has run on one stage ever since. Document legalisation for Japan therefore comes down to a certificate from the FCDO Legalisation Office, with nothing bolted on behind it. The agreement was made between governments, so it holds whatever the paper is and whoever is asking for it. That long-settled position is why this route rarely changes and why advice about it dates slowly.
Two forms of the same certification
The Legalisation Office issues apostilles by post, and it also issues them electronically as UK e-Apostilles. Both come from the same office and describe the same thing about your document. What separates them is physical: one is fastened to a sheet that must be sent and returned, and the other never becomes an object at all. An apostille issued in Southampton for use in Japan can take either shape, and the shape is chosen before the file is submitted rather than after.
What actually moves on this route
On the postal route the journey is simple to picture: one envelope out to Milton Keynes, one envelope back to your address, and no counter to visit at either end. Nothing else on the British side changes hands. Because an apostille from United Kingdom to Japan needs no consular stage, the sheet is never handed to a third body, and the only risk in transit is the ordinary risk of the post — which is exactly why the copy you send should not be irreplaceable.
Cost and pace on a one-stage road
The charge is £45 / document by standard post, or £40 / document on the next-day service open to registered businesses, and it attaches to each document rather than to the batch. An apostille for Japan therefore costs the fee times the number of sheets, with no later charge to account for. Timing rests on the post and on how busy the office is, so build in the round trip and treat the rest of the schedule as the far end's business, not yours.