Your local SW office if you are prepared to do it in person. Otherwise, an anonymous report will be investigated provided you give enough detail that the person can be identified and observed (their name, address, where they work, when they work, who they work for, times they leave house/come home, anything that will help the investigation) SW will not, as some people think, simply accuse the person of working and signing on at first opportunity. They have to properly build up a prosecution case first.
Also bear in mind that a person may be on a Back To Work Scheme anf legitimately "claiming the dole".