maybe its this you've seen - Unmarried Partner visa
"It has been possible since 1997 for unmarried partners both in a heterosexual and same sex relationship to gain entry to the UK using an unmarried partners visa. Applications for unmarried partner visas tend to be more difficult than those based on marriage as you need to show that you have been together for two years and that it is a permanent relationship. The requirements since April 2003 are as follows:
You have to show that any previous relationship whether in marriage or as an unmarried couple has ended.
You should have been living together for at least two years in a relationship similar to marriage. You will need to provide documentary evidence confirming that you have been living together for this time. "
how can you prove your marriage has ended ?