You are not the only person who remembers the recommendation letter format. I also have these memories. And, if I am right, it was in one of the Tech Nation Guidelines (around Spring 2017 or that period). So if you can find this or earlier version, it will be, probably the answer to your question.
But most people will not do this research.
Hence, I can suggest a couple of essential parts included in the Letter of Recommendation:
- self-introduction of the person who provides the Letter. This part has to demonstrate the seniority level and describe the business of the person to understand how relevant his or her expertise is to the knowledge of the candidate;
- relation between your signee and you. How you have met or been introduced and why your link is not just a networking contact;
- what projects of your portfolio are inspiring and why (in your signee’s opinion);
- how the projects from the previous paragraph can positively influence on the UK’s technological ecosystem, what is the quick (or slow) win;
- why your future move to the UK will be positive for you;
- notice about the readiness to provide any extra information. This is very important - to give full contact details, including mobile phone number and personal email. Tech Nation has to have this information for future reference and checking the details of the letter of recommendation.
Additionally, you can read this blog post, and it also provides ideas on how your letter of recommendation must look like.