Not as cheaply as Littlewoods/ Index sold them, but it doesn't look as if they plan to restock so this source may be useful to some.
Robert May Shopfittings based in London sell 12 zinc coated aluminium look grids for £19.95 + vat (item number mgp100). They do mail order and the delivery charge per item is £7.50.
www.robertmay.co.uk/cubes_systems.html Contact 020 7272 5225 (business hours) 0800 371566 (24 hour catalogue request line). They are currently out of stock but are getting more in about 2 weeks and have no plans to discontinue the line.
If you have a Poundstretcher near you, it may be worth checking there as some sell them (called House and Home organiser cubes).
Remember if you find another source in England/ UK, post it on the Cavy Cages website so other people can be helped to find them
www.cavycages.com/ (scroll down to 'Found mine at' on the menu). You can also check there for new sources others may have posted about.