The problem is you don't have a bank account I guess, either that or sufficient time left on your current stamp. You can go to Laos or any other nearby country and apply there without any problems showing funds in your home bank account. Once the visa is issued return to Thailand, open a bank account and put a minimum of 800k baht in it once your back. The 800k needs to be in place for 2 full calendar months before your 90 day Non-O visa expires so you can apply for a 12 month extension of stay
if you bring it into Thailand while a Thai tax resident then Thai tax rules apply and you will have no UK tax to offset against Thai tax. Basically the same as above
indeed, but there may come a time when immigration may deny entry. And also there is the requirement to leave Thailand every 90 days and stay out of Thailand until you receive the visa
Yes it is if you are a Thai tax resident when you bring it in. The lump sum is tax free in the UK but is viewed as income in Thailand and Thai tax is applicable plus you have zero UK tax to offset against it.