Is it okay to lie on the application? No, it's not. Declare the previous relationship. If you don't, and CIC somehow finds out, then that would be misrepresentation. There was a couple on here where one spouse did not declare a previous marriage which had been done under traditional rules, without any documentation. CIC found out and refused the application.
If you don't have any proof of severance, write a note explaining that and attach it. Explain that the relationship ended at X time, and that you no longer have contact with your ex.