I believe AIS has just brought out 3bb making them the biggest provider. I have always used AIS and they have never let me down. Wife has dtac and as soon as she hits the countryside has problems
On this matter please expose the banks that our demanding you take insurance with them. Let's name and shame. Name and address. This is not a requirement by law and can be quickly rectified by mentioning you will seek legal advice.
Stay clear from agents you don't need them. Have you been supporting her monthly? You can get her bank statements to prove this. You will need to prove you can support her for her visit to the uk. Some money in the bank and or your salary coming in. Your residence papers in the uk to show where she will be staying. And a big reason for her to return to Thailand after her visa expires this is important. You don't say why you're first application failed because this can have a big bearing on any future applications