The reason is simple, ask ayg, it is due to Britain having been taken over by foreigners. A British passport does not make you a British citizen any more. The once defiant nation has surrendered to the foreign hordes. As noted by an amerian journalist and eye witnessed by ayg, London is now an islamic state.
A simple frisk and body scan is no longer a safe option at the british embassy. Who knows what you have in your shoes or body crevices?
You are not welcome there. The letter with its pretty embossed seal of approval is issued at a fee greater than the cost of annual Thai visa renewal. The wise Thai do not issue an extension without a personal interview. A good check on whether you are who you say you are.Not so the great brits.You won't encounter one, a true brit, at the bullet proof window where you can try to communicate with the embassy staff. No, foreigners do that job. Ironic.
Due to these foreigners taking over britain, the pound has crashed (brexit) and the exchange rate fluctuates daily. Hence a postal order issued in Thailand might be worth a few less (or more) baht than the exchange rate at the time of payment. Also selling off the once beautiful gardens a few quid were made. Not allowing british citizens into the compound saves on staff and a few more quid made. And let's face it the only reason you are not in blighty is to escape the foreign hordes who populate the once "Green and pleasant land" You can not be trusted not to swindle a few baht from your country. Very sad, Churchill, Palmerston and the like are turning in their graves Queen Victoria is "not amused'.