Although, they do not have them on their website, Bikefix http://www.bikefix.co.uk/
are an arkel dealer in the UK. Perhaps, if you ring them, they will order you some in.
Otherwise, you can get them by import from Arkel's own site, here is the link:
If you can get them from Bikefix, can you let me know- I'm considering getting some. Don't forget to get waterproof covers too if you're planning to cycle in the rain!