Thank you for your enquiry about your need to isolate for 12 weeks.
Please follow this link to the Government guidance. This guidance covers the whole of the UK population and not just those who have been sent a letter by the government.
The primary purpose of the letter sent by the government is to identify the 2% of the UK population who are at critical risk of infection. The remaining 98% are still at risk and need to manage this risk alongside their employer.
You can decide if you need to self-isolate and discuss with your employer. In the first instance your employer should offer you remote working. If this is not possible then you will need to consider whether you fall into the furloughed employee category – more information on this link about the job retention scheme
Because you are well enough to work, however unable to attend work because of the risk of contracting Corona virus, we are unable to offer a fit note to cover this period. You will need to work with your employer to resolve this situation.