I saw some really skinny stuff in Homebase when I was in there on Sunday. To be honest, it doesn't look very solid, but then I'm not sure what sort of thickness should be used really.
I've just been on the Focus website, and it's a bit pants
I tried a search for plexi, but it said that this item isn't available on our website.
Google isn't much help either, but I suspect a more careful search may turn up some results.
If you do find a supplier, please post it here, as I'll be shopping for some myself
Oh, whereabouts in the UK are you? I'm just outside Cambridge.