I pay 3500 for a visa run from Pattaya. I don't recall the name of the visa service, maybe First Class Visa (not sure). They operate out of the Victorian on Soi 6 Pattaya Beach Road. I don't know if you're in/near Pattaya.
Someone in this group will have a recommendation for whatever area you're in. Time/Distance will effect the cost.
If you want to sleep in the temple and eat only rice you can probably get by on a dollar a day. If you want to live in a high rise building an drive luxury cars it will cost considerable more.
Recently someone asked a similar question.. If 2000thb/day is a enough to get by. That seemed like a pretty good number.
I transfer 65,000 every month, of that, 20,000 for rent, 5000 for bills, leaves 1300thb/day. That's enough to get by but not enough for the incidentals, traveling for visa runs, travelling for recreation, lost cell phone, etc.
Bahtsold.com is a pretty good site to find rentals, to give you an idea about what you can rent.
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,
*****
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.