You will have to compare rates, but most travellers exchange at the airport in Japan or at big banks. Post offices also offer reasonable rates and often accept foreign debit cards. Last time we were overseas (travelling to Russia) we exchanged JPY into RUB in Narita and later found out that local exchange rates were far better.
Noticed the British flag under your name. If you have a Nationwide debit card, it can be used in the many post office ATMs in Japan at a reasonable exchange rate.