Sockets in Japan
Type AType B
Your the UK plugs don't fit the sockets used in Japan.
Japan from the UK means two changes at once: plug shape and voltage. Japan uses flat Type A/B pins, so your UK Type G fits nothing, and it runs on 100V against the UK's 230V, the biggest voltage drop a British traveller will meet.
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No, you need a Type A adapter.
Unlikely. Most are single-voltage 230V, and Japan runs on 100V.
Types A and B, at 100V / 50-60Hz.