You will be asked at the border ...why you live in Thailand longtime without a longterm visa. They may assume you are working illegally and say get a longterm visa before we let you in.
You did supply all details of her mother,father, daughter in applicaion, such as dates of birth ID card numbers. Copies of her bank statements. Letter of inivte from you stating your address, copy of your passport, copy of your own bank statement showing you can support her for the time in the Uk. List of potential place you would viist in the UK. State you will pay for all flights. Dont ask for 6 months just give estimated date of arrival and say only staying for a month.