Put quite simply, there is no legal limit on the quantity of alcohol or tobacco you can bring back from an EU country, provided that it's for your own use. (This doesn't allow you to bring it back for friends etc)
Following a court case (involving Hoverspeed) C&E must have "just cause" for stopping you, such as intellegence to the effect that you are a "regular traveller" bringing back large quantities, you are known to be selling your wares or that you fit a particular profile. You are quite at liberty to ask why you have been stopped.
I believe that there is another case pending.
I had a case where a guy was stopped, after leaving Dover, by the police because his van was overweight. C&E pitched up aand seized the van and its load. Where was the just cause in that - they had let it through the official checks at Dover without question.