How soon after having a visa , can a Thai apply for another visa to visit U.K.?. It was a 6 month visa she had but only applied for 28 day stay. So only stayed for length of time applied for, can she apply for another one before current one expires?
TLDR : Answer Summary
A Thai individual holding a valid 6-month UK visitor visa can apply for a new visa even before the current one expires, provided she departs the UK prior to the expiration date. It's important for her to justify her second trip when applying for the next visa, as the application will be assessed independently of the first.