The only 'contraband' you've mentioned is the cheese and you've learned the limits for that.

If you have room you might like to take some twiglets. I once took loads of English 'delicacies' to Tokyo for a big pre-wedding party to celebrate me & MrLapLap getting hitched.

The twiglets (essentialy marmite coated shards of bread) were particularly popular. I also had a case of exotically flavoured crisps; pork & stuffing, turkey & cranbury, beef & stilton etc... The snack that took our friends the longest to get into were the cheese footballs, they needed to be a bit drunk to appreciate their junky charm (there's good reason why the Brits only eat these at Christmas!).

And if he wants to introduce anyone to marmite then there's this infamous recipe recently championed by Nigella Lawson for Spaghetti with Marmite which has an uncanny resemblance to spaghetti with shoyu-butter, a classic Japanese pasta dish.

If you actually want to gross out a Japanese person forget marmite, get them to try a Cadbury's Creme Egg.