No real input from me, I'm sorry to say.

The Duty free shops in Spain were always more expensive than the shops outisde the airport - even in the old days when Spain wasn't part of the European Common Market. I guess this trained me from an early age to be suspicious of Duty Free shops.

When looking for British things to take to Japan or Spain there's nothing in the London airports that seem particularly interesting, let alone interestingly priced.

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The turron in Spanish airports is always cheaper elsewhere, and the bottles of olive oil always seem a bit of a rip off (I don't even rate them more highly then some of the better olive oils available by the litre and in plastic from Spanish grocerers - more details in olive oil thread).

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Narita is interesting as some of the duty free shops allow you to sample a range of their sake before you buy it. I'm not sure if the sake is much cheaper than that at stores outside the airport, but having the chance to try it first is wonderful. Although there are certainly places in Tokyo that also offer this opportunity (in more relaxing/amenable surroundings) they aren't the sort of places you'll just stumble onto, so Narita can be an excellent place to buy sake for those of us who aren't knowledgeable connoisseurs.

A food I've found easier to source in Narita than in Tokyo are large umeboshi (a dozen or so will cost at least 3,000yen). The thing is there is a bewildering variety of speciality foods at NRT. Even if I devoted a year (and a fortune) to trying them all, by the end of that year many of them would become unavailable and there would be a whole slew of new items to replace them. And they change constantly to reflect the seasons.
One of the few foods that is (or should be) constantly available are the cookies from 'Yoku Moku'. They are also available in some of Tokyo's department stores. Yoku Moku is quite well known, so someone manning an information desk will probably be able to help you find it. http://www.yokumoku.co.jp/ (apparantly they sell in the US, so this might be more useful for us Europeans)

Also of note are the white chocolate 'cats tongues' from Sapporo based Ishiya
http://www.shiroikoibito.ishiya.co.jp/ which can/could also be found at NRT
Chocolate blanc de langues de chat (I've also had the milk chocolate ones, which were nice - but the white chocolate ones really were special)
http://www.shiroikoibito.ishiya.co.jp/goods/index.html
- This company were a bit naughty with their labelling last year. I'm sure they're extra careful now!

Narita, the area, is famous for some of its own local specialities. The main ones are listed here: http://www.city.narita.chiba.jp/engl...delicious.html
I highly recommend the Teppozuke.