I just did this for an ED VISA, emailed all the docs to Hull they check then send to London, then London confirm with hull and then you go to Hull in person with your passport and other docs and pay the money. Was very straight forward. Took less than a week.
It depends on the type of job, if its a teaching job you need a degree if another type of job and the company can provide a work permit and visa you will be ok. You could ask the company if they have provided this for someone in your position first. I know people that have a job/visa/work permit and dont have a degree. Id make sure you sign a contact with the company so you know how long you employment will be.
Same thing happened to me a while back changed the timetable on certain days.. I had to fly to udon to make my appointment. it worked about better. Thai Lion air have an early morning flight from Don Meung to Udon---mini van to the border.
Heres what you need to cancel your visa, this was in March this year, it was a lot more than a letter from my employer, I needed documents relating to my work permit from the tax office. Its in Thai I was given it at Chang Wattana. Hope it helps .
I was told I need to take a test after the first 3 months to get the first extension, this was a change that had been brought in, but then heard differently and some people say no test. They are a lot of changes that have been brought in, Im not sure what has happened with you. I would get your school to look into it a bit more as it will most likely happen again to someone else. Hopefully someone on here can help you out. Good luck.