As long as you paid already, then it should be within a week. London is usually quite fast. But sometimes if they have a slowdown they prioritize processing applications based on departure date, so if you put a date far away, they would not prioritize your application and it could take longer.
It's not (or the equivalent amount internationally). The reason no one gets this visa is because you must have 3 million in a THAI bank account to apply, unless you're Australian and applying through the Sydney consulate.
Departure information and visa number are not mandatory information. Don't touch any part of the departure information and you don't have to fill it out. Touch any part of it and it all becomes mandatory.