In America, the question is 'which church should I go to?' because everybody goes to church.
The answer is, 'Come to ours, because we are less conservative, less dogmatic, because we are not like churches that you dislike.'
In Britain, the question is, 'why should I go to any church?' because nobody goes to church.
Our answer has to be to say what is good about being religious. What differences being religious makes to our lives. We need to rediscover what is at the foundation of our religious faith. How we are made better people by this thing that we do. We need to say what we are, not what we are not. We need to say being Unitarian makes a difference in our lives. If it does not, then why bother?