I'm expecting everyone who isnt submitting a tax return in their home country to have ALL money they send to Thailand taxed by the Thai authorities before long
Has to be obtained at your Embassy in Thailand. If you been previously married you will need the divorce papers. If your former spouse is deceased then you will need the death certificate.
Any property owned get a translation of the deeds. Get copies of her bank statements and bank book, state who will be paying for the trip and put in your own bank statements showing liquidity. Hand write a letter stating you will buy return plane tickets and support her during the visit , give expected dates of arrival into the UK and estimated date of departure, sign it. There's a translation and copy service provided at cost on the day at VFS.
If you haven’t got your visa yet. You will need entry ticket into Thailand, entry ticket out within 60 days AND. another airline ticket showing your next flight into Thailand