No problem taking it on in the UK. Might be a problem if you want to take it off at your destination. The US,Canada e.g. are very strict on food imports even it they are sandwiches not consumed in flight.
A teacher friend of my sister in Canada tried to bring back fresh sausages from her parents village in the UK . Canadian Customs said she had to surrender them so she threw them on the floor and danced all over them. She picked them up and handed them to the bemused customs officer and said,"If I can't eat my sausages you sure as hell won't either".