Why not traditional things from here like a teddy in a union jack t-shirt, badges, book covers, a calendar etc
A compilation c.d like Now 77 or whatever the number is.
Those little dolls in national dress like a beefeater.
Stuff with royalty should go down well, foreigners love the idea of royalty.
Small piece of Wedgwood?
Harrods teddy or anything from Harrods?
Book of aerial photos of the UK?
Christmas stamps set?
Commemerative coins?
M&S key ring (model of the new mini)
A small (not full size) Royal Doulton lady - Christmas Morn or similar.
Shetland fudge
A Burra bear (Fair isle knitting) - see their website
A Welsh doll , a Scottish doll and a beefeater
A Halycion days enamel box (depends on how much you want to spend really.
Beatrix Potter stuff
Sadly, with the global economy there isn't much you cannot get anywhere in the world these days! There is a Japanese shop in Chester that sells all sorts of Japanese gifts.
Consider tea (and other food / drink such as shortbread)- but check the rules about importing into Japan first. Whittards or Fortnum and Mason do nice presentation packs, and the Japanese like tea