Have you been on the Retro Raleigh site for the dating and other info? The 501 tubing sounds like it's a newer Raleigh since they reused some of the names on Raleigh USA bikes(or are replacements), however the Carlton label certainly would seem to make it an English made bike. Does the headbadge have "Nottingham" or a series of X's beneath the Heron? Is the bottom bracket marked for brand and thread size? Are the dropouts marked? Deraileur hanger or claw? These details would help to figure out what you have.
Pete Geurds
Douglassville, Pa