[Nyororin]

It`s usually best in the end to change your money through a *bank* and then take it with you. Airport exchange rates are atrocious. I can`t imagine you ending up with more money if you went the Pounds->Dollars->Yen way, as there are fees at every level.
You can`t actually use Dollars in Japan, so it definitely seems like a waste of time and money to me.

Credit cards work in Japan (if they`re international type cards) but debit cards are quite iffy. I believe some work, some don`t, and some appear to work but charge massive fees. It`s better to be safe than sorry.