As much as I disagree with that antiquated establishment view of relationships, I agree entirely that it will much harder trying to apply from a boyfriend/girlfriend point of view. A couple can marry out of convenience and live apart for chunks of the year - they would be in a much stronger position with their application. Fair? No, but the fact remains that it is much easier to provide the evidence of a marriage certificate than it is to supply evidence of a "durable relationship for in excess of two years" as an unmarried couple.
I think that the OP needs to bite the bullet and marry - cheaply in Thailand, then apply or (my favoured route) in the UK on the back of a fiancee visa. The later has a good chance of success with just the simple criteria for a standard visit visa, plus a registry office booking/enquiry.