There is nothing to the best of my knowledge in the Defra regulations preventing the feeding of ale slops to pigs. At no stretch of the imagination could a bar or a cellar be classified as a kitchen, so carry on.
However, a small cautionary tale. A late friend of mine used to farm in Warwickshire and kept Berkshires and used to have visiting sows to make use of his services (the boar's). One couple arrived and off-loaded their sow from the trailer. The next morning, the pig was decidely off and as the day progressed got progessively more bad-tempered both with the boar and my friend until it got to the stage that he had to remove her from the boar's pen as he feared that she might do serious damage. She continued in this vein for several days and the farmer came close on several occasions to calling her owners and demanding her removal even though no mating had yet taken place. By day 4 her temper was improving and the next day she was returned to the boar and all went normally.
The sow being in-pig, her owners came to collect her and my friend related his strange experience when she had arrived. The couple looked at each other before confessing that they ran a pub and that every day she would get as part of her rations all the ale slops plus the dregs from all the glasses. She must have been in a permanent stupor until arriving at the farm when she suffered a hangover and cold turkey.
So be warned!