The app is available in the UK store, but Apple's search indices may not have been updated. Try buying it using the direct iTunes link on Quip's website, instead of searching the UK store.
That's strange, I tried it earlier using the link on the website which took me to itunes, and it was listed as not available in my store, but perhaps they fixed the link (might have been a link to the US app) or it was a cache issue as you say. Seems to work now.
