How long before traveling in December should I apply for a Thai 60/90 tourist E visa from the UK?

The Thai tourist E visa is valid for 3 months from the date of issue, so it's advisable not to apply too early. Generally, you can start the application process around 1 month before your trip, as the embassy typically processes the visa in 3-5 days. Ensure you have all the required documentation to expedite the process.