First of all before you do anything you need to speak and get advice from a good independent lawyer. By that, in Thailand, I mean a lawyer from the city not a family friend of your Thai girl friend or her Thai family. The following is meant a general guide that you can use as a prompt to discuss the matter with your Thai solicitor. I would also say that however much you love and trust your Thai girlfriend it is imperative that before you marry you take all steps to protect your assets and take good legal advice on the Thai law surrounding marriage in Thailand.
Although it does not always guarantee automatic protection from any future claims by your soon to be Thai wife if you get divorced keep properties, land and assets in your own sole name,.
Keep your bank accounts, savings and finances completely separate from your Thai wife so that, if you get divorced and if you need to, you can show to any divorce court that your Thai wife did not need all the money that you had access to live on and that you both intended when you started out in the marriage with you to keeping your finances separate and for your asset to be seen as your own property and not to be shared or communal assets.
Thai wives should be encouraged to earn, make and spend their own money so the relationship looks like a partnership and she is not depended on you for her needs.
Remember if you get divorced from your Thai wife in England and Wales there is nothing (not even a pre-nuptial agreement) that can guarantee protection of your assets in the UK. The longer you are married and the more children you have together the more generous UK courts are likely to be towards your Thai wife in any financial settlement.
If you both live some of the time in the UK and some of the time in Thailand and you want to get divorced, speak to your Thai solicitor about any advantage in starting divorce proceedings in Thailand. You may find financial settlements in Thailand are to your advantage, especially if you have a good Thai pre-nuptial agreement that must have been signed at least 21 days before the wedding.