After you got married, you need to apply for Further Leave to Remain or FLR(M). If you want to pursue the marriage on 25 Aug., you need to make sure you have all the documents required for the FLR(M) since it's just a week or 2 (?) before your visa expires.
Go to this site http://www.ukba.homeoffice.gov.uk/vi...pplying-in-uk/ and read about the requirements and process for applying FLR(M).
