I've seen several posts asking for bento sources in the UK, so I thought that rather than replying with my tips individually, I'd do a quick round-up:
(1) The Japan Centre (goes without saying, really) - this is moving to Regent's Street next month. I find tJC a bit odd with some items only available online, some items only available instore, etc. But it's a great source for boxes, chopsticks w/case, forks and spoon sets, dividers, egg moulds, onigiri moulds, sauce bottles etc.
(2) Paperchase! At the moment Paperchase is doing a really cute range called "Funny food" and the items would make great bento accessories without costing too much. The design has changed this year, with the earlier design (I'll call it design 1) being a little less Japanese in style, red- and green-based, and the later design (design 2) being very kawaii looking with blue and pink bases. Here are some piccies of design 1 in action: http://printpattern.blogspot.com/2009/03/paperchase-funny-food.html  and http://www.ohdeedoh.com/ohdeedoh/meal-time-goods/kids-tableware-at-paper... . The range includes all sorts of things like:
- water bottles (these are plastic and only £3.50 each)
- food containers (not lock/lock style so good for dry foods only)
- fork and spoon sets
- tea towels
- stickers (would be cute way to jazz up some wrapped onigiri!)
- recipe books (I use mine to copy down internet-based bento ideas to take into the kitchen with me)
- magnets (I'm using mine to stick the bento meal planner up in my kitchen)
And so on! Items with design 1 are still hanging around some stores, although most seem to be design 2 in the last month or so. Paperchase.co.uk is selling the old design.
(3) MUJI do some very very plain white or black bento boxes (with useful clip-style lids) and chopsticks w/case.
(4) The Whole Foods stores are carrying a range of adorable lunch boxes and useful bits and pieces from "Rice" which is a Danish company. I've seen these lunch boxes: http://www.rice.dk/ProductPage.aspx?id=9371  and also some which are blue and have a fox/squirrel design. (the website has some very girlie ones, too, which I haven't seen on sale). They aren't lock/lock but do have quite a firm closing action and are very deep. These little boxes: http://www.rice.dk/ProductPage.aspx?id=10351  are also lifesafers if you fancy chucking in some seeds, raisins, anything you don't want shaking out all over the rest of your bento. They come in squares, rectangles, rounds and flowers. They have quite good closing action and can be stacked up in a deep box (like those above). The boxes are £7.99 for the small one, £11.99 for the large and then £4.99 for the 12 mini boxes.
(5) Anyone who's looking for plain Lock & Lock in the UK, the best selection I've seen is Lakeland (www.lakeland.co.uk ) which doesn't have a store in London but has several elsewhere. They also do little useful bento things like mini ice packs, thermos flasks, cutters for making your sandwich into a dinosaur, etc...
Anyway, those are my top tips for bento in the UK - hope they're useful! I'm going to Hong Kong next week and will be keeping my eyes peeled for useful items I can re-purpose for bento making :)