After the update on the London E visa application site, I've seen a report that the Embassy and consulates are specifically asking for the letter from your employer to be addressed to the embassy or consulate when applying for the METV. Somebody reported a successful E visa application but it was noted the letter wasn't specifically addressed to London

I found that the Liverpool consulate has updated it's requirement to reflect

5. A letter from the applicant’s employer (in the UK) and addressed to the Royal Thai Consulate) stating length of service and expected return to work. If you are self-employed your self-assessment form.