yes you have to be careful on selling up and moving, with regard timing. Under current system making the move from July onwards has more tax advantages. The 150k bht is a yearly figure not monthly.. checkout the various webinars at expattaxthailand, very useful info.
incorrect, it doesnt matter about tax residency in another country, thats why there are DTA's. If your 'Assesable Income' is less than 120k bht then you dont have to file.
If your a tax resident still in uk, you shouldnt have to pay tax here (as you have already been taxed), but filing is how this is communicated and credits used, and under certain scenarios tax may be due.
Filing a tax return for tax residents with assesable income (remitted or earned in country) of more than 120k bht in the tax year is mandatory, so no requests are sent out. This is not a new rule and has been in place for many years. Filing allows you to reduce or negate the tax owed using the Thai allowances and DTA credits. Not filing doesnt mean you dont owe tax, and it would be much harder to gain credits against for historic years.
Well done on the visa, but on a sour note, you've told the government you intend to live off rental income which means you supposed to file 2 tax returns each year if you stay 180 days or more instead of 1.you shouldn't actually owe any tax as you can credit off the tax paid in the UK as per the DTA.
Life will be a lot simpler if you sort it out before you leave, the 65k route to getting a visa isnt an option for UK passport holders. I'd at least get one while in the UK, and then you only have to worry about the spouse visa, which if you cant get in the uk, you can sort out here easier as you already have the visa to attach it to.
At a guess your UK passport holder, so you can only go the 800k per main visa holder to qualify for visa and first extension. To move to the 65k monthly method in the 2nd year, you need also to transfer 65k+ from 1st month of moving so you have to maintain both methods for the first 5 months. Then just 65k+ for the rest of the year to get yr 2 extension.