What are the requirements for the work contract and proof of address when applying for a DTV1 visa from the UK?

Contract should, as mentioned above, state you're able to work remotely. In a recent application via London, further information was requested; they asked for work contract even though it had already been submitted. I saw that as a request for clarification of the remote work status/permission, and a further letter from the person's line manager as a specific response to the embassy, dated the day after the request for more information, explaining that not only was the person able to work remotely as stated in their contract, but they had permission from the company to work in Thailand. Also mentioned was that the company has no Thai clients, and derives no income from Thailand.

Application was successful after submission of the extra documentation.

The requests seem to have a generic style so I see that if they're asking for something you've already submitted, they want more detail.