When I worked in London itsu was a particular favourite of mine for a gluten-free lunch option. There's quite a few branches dotted about London, I only wish they'd open up in Manchester!

Whenever I'm in London I usually end up having lunch there - the 'Super Salmon 3 Ways' box is a particular favourite; although the soy sauce is not gluten-free so you'll have to do without that.

Itsu actually offer a rather comprehensive allergy/dietary information sheet on their website so be sure to check that out before ordering.