I sent my passport to an agent last year, who did all the work for a reasonable fee, including the payment to the Passport office in the UK, and it was back along with a new one in 3 weeks. If IIRC the charge was about 5,000 baht and far less traveling time.
My travel costs were the 6km journey to the Thai post office and back once instead of getting my wife to drive me 350km each way to BKK plus an overnight stay, twice.