But METV is a TOURIST visa. That means it's for tourists. Whether you have a visa or not, immigration is the one who decides if you get to enter Thailand. And if you just spent a potential 9 months in Thailand and try to return as a tourist 3 months later, they're likely to take a dim view.
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.