Perhaps you've simply not spotted it because you're looking for the old packaging? It looks quite a lot different now!
http://www.horlicks.co.uk/products/
Chris, Ive scoured the shelves in my local sainsburys and looked all through the tea, coffee, hot chocolate drinks section, then asked customer services who said they have not been able to get stock due to the packaging change over...
Well it was definitely on the shelves (although there were only a couple of packs) in the Stanway (Colchester) branch of Sainsbury's earlier this evening. I made a point of looking for it!