I don't know the charges, as my UK building society absorbs all of them for me while I'm abroad, but TM seems to know what he's talking about so I'd guess he's right about that too.
You'll need to let your bank know you'll be in Japan, so it's better if you go in and speak to someone and then at the same time you can ask whether they make their own charges for international withdrawals, and ask them whether the Japanese companies might make charges. My building society told me they'd absorb the Japanese charges and not make any themselves, and it's been true. If your bank says otherwise, ask around a few other banks to see if you can get a better deal with their card.
Oh, and if you won't have to pay charges you will be better off getting your yen here. If you do have to pay charges, get as much as possible in the UK and then use the ATMs in emergency only. Many places here don't accept credit cards at all, not even Japanese ones, so don't assume you can pay with plastic everywhere (regardless of what they say on the adverts.)