Below is a link to the Thai Department for Disease Control's dedicated website where you will find the latest information regarding the coronavirus outbreak and associated travel advice.
For travellers from the UK, the current advice is:
1. Travelers with fever and at least one of the following symptoms: runny nose, sore throat, and difficulty breathing must notify disease control officers at the quarantine office immediately upon arrival in Thailand.
2. All travelers will pass a fever screening through a thermal scan. Travelers who meet the criteria of surveillance will be referred to a designated hospital.
3. All travelers are recommended to implement control for observation (meaning supervision without quarantine) in order to ensure self-monitoring at their residence for no less than 14 days and comply with the recommendations.
Source: Thai Department for Disease Control's dedicated COVID-19 website
NOTE: If you are transiting through airports outside the UK you should check for any travel restrictions or requirements that may be in operation for passengers passing through those countries.