Hey Sophie!
I understand your troubles, living in the UK it took me a long time to find the places where I could get my kawaii fashion fix! These places are hard to come by. I'm glad that you've opened this thread tho, so that us Brits can get together a bunch of links and addresses for places that will stock the stuff we love!
Apart from the others already suggested, a few places I know are:
ChickiThunder EggTokyo Jazz Panda
They're all online stores that specialise in kawaii style. They're better for buying fashion accessories rather than clothes, but they're always changing they're stocks so cool clothes items do come up from time to time... so definitely worth keeping your eyes on these places.
I don't know where in the UK you live, but if you're up north then you can visit the real Thunder Egg store in the Northern Quarter in Manchester. It is an absolute kawaii wonderland in there! So much more choice in store than online. Tokyo Jazz Panda is also a real store which is in Chester. I've not managed to go check it out so far... when I do I'll make sure I report back!