a few years back, whenever me or my co-workers travel for business trip in Japan we carry most of our money in travelers cheque, because we get better deal with them.
we usually just exchange enough for train and food in the airport and do most of exchanging in Akihabara. Laox or Yodobashi Camera have money exchange counters a their rate is a bit higher than banks especially on a weekend
I have roamed Tokyo and Osaka with 500yen in my pocket or more than several hundreds but I have never lost a single yen yet ..
Travel Insurance? I do pay them, but most of the time its included with airfare. but am not sure what is the policies in the UK though..