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.
No Thai embassy requires 800k baht. The DTV requirements are 500k baht in a bank account. A Thai bank account is not required and is infact impossible to open without a long-term visa.
Each year you can apply for a brand new extension of stay. You cannot renew anything.
If you applied for a 1-year non-OA visa from your Thai embassy which requires insurance as part of your application and then used that to enter Thailand, each extension afterwards will also require insurance.