in November 2014 I complained about a Visa issued incorrectly by Bangkok British Embassy to the Home Office "on-line" complaints contact, refer to 2. above.
Within 7 days I received a detailed denial from 1. above.
i suggest you initially send your complaint to 2. above so that it is logged in London. I think they will automatically forward this on to Bangkok from where you should receive a reply within say 7 days? If not then send DIRECT to Bangkok, refer to 1. above.
Hope you have better luck than me (supported by my MP)