In my Philippine passport I have a marriage visa for the UK valid until the end of the year. May I return to the UK on that visa before it expires? Me and my fiance haven't had a chance to get married in UK as because of unexpected that he needs to go to USA for his work, that's why I am back here in Philippines, but we still have a plan to married here in Philippines.
From the further information you provided in the comments, all of your questions are given in the rules...
You have a multiple entry clearance and you have NOT yet married (and presumably you have a good reason for not marrying during your last visit). So yes, it's ok to enter again. This is stated in Paragraph 20A of the rules. Had you been married, then no, that would make you an illegal entrant under Paragraph 30C of the rules (entering for a different purpose than that given on the entry clearance).
Don't expect them to be happy about it. To be fair, you can expect a thorough grilling at your landing interview on why you came and did not get married and why you think you'll get married this time. If you do not think up something that satisfies them, you will face detention under Paragraph 30A (ii) of the rules. Also, if your plans have changed and you do NOT intend upon marrying during your visit, they will be entitled to revoke your entry clearance on the spot under Paragraph 10B of the rules.
So, yes, you can come again and stay until your entry clearance expires IF you have a successful landing interview.
The rules are at https://www.gov.uk/government/collections/immigration-rules