If you have evidence that electoral fraud has been committed you should contact the police as quickly as possible using the non-emergency number 101. You should be prepared to give them a statement and substantiate your allegation. If you are worried about giving your name to the police, you can also report electoral fraud anonymously by contacting Crimestoppers online or by phone on 0800 555 111.
If you think electoral fraud may have been committed, you should first speak to the Electoral Registration Officer or Returning Officer for your local area. For Southampton the number is 023 8083 2245. They may be able to explain whether or not electoral fraud has been committed, and can refer your concerns to the police if necessary. They can also provide you with the details of the police contact for the relevant police force so that you can report the allegation yourself.