Why Do Some Supermarket Checkouts In The UK Sell Alcohol And Some Don't?

This is a hugely frustrating annoyance that annoys me every time I forget.  One can get to the front of a long queue before one remembers or is told that one can't buy a bottle of wine at this till.  Surely if the building can sell alcohol, it could sell it anywhere.

But actually, this is not the case, only a certain amount of the checkout tills are licensed to sell alcohol.  This begs the question, why not license them all?  

I'm not sure of why they don't allow all of them, it make sense that if the building sells alcohol that it doesn't really matter which part of the supermarket one buys it at.  However, it appears that it does.

One should look out for large ticks and crosses near the checkout which tell you which tills may and may not serve you alcohol.  Either way it still seems rather lunatic.