If you break the law of overstay and don't get it cleared before you go to airport then you will be banned by airport immigration... there's no reason for anyone to be on a overstay before arrival to airport... check the law before doing the crime..always clear any overstay before leaving the country....or you can get a 5 year ban for a 5 month overstay.
You don't need a agent if you got the money in the Bank.. practically easier paperwork for you than you did for your gf first visa to the UK ..your get everything you need by contacting the Thai embassy London ...