Most large Superdrugs.
Surrey Quays Superdrug does for sure. I'm sure other large Superdrugs do.
Btw I don't rate the products much. I bought the original i palette eyeshadows after lots of rave reviews. It's ok, but nowhere near as good as my trusty Urban Decay shadows. Haven't tried other Sleek products.
Also 10% off Superdrug w NUS card