I believe the best option is to get your retirement visa here. Come on a tourist visa then work out the retirement visa. You will want to ask the group this question because I am not so familiar with UK Citizens, but for US there are different insurance requirements and its more favorable to get the visa in Thailand.
You can survive comfortably on 40,000 but it won't leave you much to do other stuff.
Why bother with the ED visa? You can learn Thai, you can go to class, without the visa. The ED visa is fine but the last I knew, they require you to go to class and take tests to keep your visa. Some people have reported they've had issues because they had an ED visa. An immigration officer once told me ED visas after 40 look suspicious. That doesn't mean much but if you can do without a ED visa, why do the ED visa.
Multiple Entry Tourist Visa (METV) is a good option also. They allow you stay in Thailand up to 90 days at a time. Every 90 days you will need to exit Thailand but you can come back 10 minutes later.
Retirement Visa require you are 50+. If that is a couple of years away for you then you can get by on a couple of METVs. If you are several years away, you will have to get creative (including ED visa) but I find it's worth it to live here.
You have to show either 800,000thb in a Thai bank or you have to show a year's deposits into a Thai bank of 65,000thb every month.
I have a buddy that uses retirement visa, he is also from the US. He tells me it's better to get the visa in Thailand than to get it while your in the US. Is reason is that the insurance requirements are different.
I think every time I apply for an eVisa they ask for more documents, I guess it's normal. If you are in country you can do the visa your self (1900 + bunches of headache) or you can pay someone to do it for you,
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thb. You decide what is more valuable your time or
Cambodia is about 3 hours by car from Pattaya. I usually hire a service to run me to the border and back. Its takes about 8 hours round trip (we stop for lunch and the border takes a little bit).
You can do it yourself but I don't think it will be any cheaper and the agents tend to be more organized.
I taught for 2 years with a college degree and a TEFL certificate. I found a company here in Bangkok that paid for the TEFL and I worked for them at a discounted rate for a semester. After the first semester my salary went up to market (45,000 - seven years ago). TEFL or maybe TESOL I don't recall.
Being a foreign teacher in a government school sucks but its pay and it's kind of nice experience. After a couple of years I went back to IT work, remotely. I have managed to stay in Thailand for 9 years.
Hence, you may be able to do something similar. Once you've been here for a bit you can meet people and maybe find other employment opportunities. You would be able to be a welder, you simply couldn't compete in the local labor market, it's illegal for you to work in a position that a Thai person can fill, also they'll do it much cheaper than you will ($15/day). But your know-how and English skills gives you a unique advantage to help negotiate building contracts.. Quite possibly you could open a contracting business.
There is no way to know why they haven't approved your application. We can all speculate but that isn't fact, not even in Thailand.
I would recommend you provide bus tickets showing you will be exiting Thailand at some point in the next 2-3 weeks (give them time to approve).
In the past when they asked me to provide additional documents they instructed me to send a an email with my name in the subject line. In that email I provide them the documentation and an explanation of my plans. In your case, I would say something like I plan to spend the next 6 to 9 months visiting the region with Thailand being home base...
They understand English pretty well but its still their second language, keep it simple, concise, and most importantly, polite. Explain you're here now but will be doing some back and forth with neighboring countries because you like Thailand so much (no harm in a little brown nosing).
When you do eventually get your METV, you can just do a border bounce and come back on the METV.
I once ask a agent how much for retirement visa and how much for a marriage visa
Retirement Visa
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Marriage Visa
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Both, of course, cost 1900 at the immigration office but it should give you idea of the complications. Marriage visas are worth while if you are actually happily married and you have an abundance of time. I found it easier just to do back-to-back tourist visa runs.