Yea I noticed on my travels that if there is more than one of the stock then probably it's in most stores. Check the two in central (Charing X rd/Tottenham Court, and Covent Garden) if you don't have any luck. I'd be surprised if there are none in stock.
Cheers for heads up, which TK Maxx were you in?
Edit. Just saw non London, cant read. Might check out Brixton one tomorrow.