From my brief conversations about Office licenses, I think that the Home Usage licenses are a side effect of the per-machine licensing policy for Office. Microsoft explicitly say that you need an Office license for the machine that you're sat at while running it, so if you're accessing a Citrix server from your home PC then that home PC needs an Office license (even if it's not running Windows). That means that the sort of places buying Enterprise agreements, with large VPNs, need a license for all of the PCs at home. So these basically come bundled once you're buying enough.

I assume that the £10 cost is just for the media (rounded up a little!). We never got as far as offering this to our users though.