After someone has died they are taking to a mortuary awaiting identification. All deaths must be registered in the country where the person died. In Thailand, a local death certificate will be issued by the local authorities. It will be written in Thai and available generally within 2/5 days of the death. You can ask for extra copies of the death certificate where possible.
You do not have to register the death in the UK. The local death certificate can usually be used in the UK for most purposes, including probate. You will need to get the death certificate translated from Thai to English. Translation services are widely available in most major cities.
If you want you may register the death with the Overseas Registration Unit. You can buy a UK-style death certificate, known as a Consular Death Registration certificate. A record will be sent to the General Register Office generally within 12 months.