ugh - Japanese marshmellows are yucky - they taste like the have aspartame in them. american marshmellows are one of my favorite snacks, and when i lived in Japan for a year i would have my mom send them over.
Japanese marshmellows usually have a flavor to them, or are filled with something gummy. i've had fruit filled ones, and yogurt filled ones, and they really don't compare to Kraft at all.
I did some searching but i couldn't find any stores that send to the UK. they're only $1 in Japan - maybe you know someone there that can hook you up?