Food is okay to mail from USA to USA.

You may have some difficulty when bringing food into the country - say, at the airport. Pre-packaged food is generally okay because it has passed food inspections, and I am sure treaties between the USA and UK treat them as acceptable.

Unpackaged foods, meat, cheeses, and produce are another matter and would have to pass Ag inspection (agricultural inspection). They used to get my mother-in-law all the time for trying to bring delicacies back from Greece.