It’s not mandatory however. They can’t actually stop you from working if you wish to do so. Can’t your employers make ‘reasonable adjustments’ to minimise risks at work?
@Stitch147 clearly hasn’t required a fit to work letter. The shielding period is only until 31st March (unless it’s extended) and the letter from the Govt only covers to that period not when the vaccinations have been completed. Hopefully things will start to open up and normalising to some extent by April. I’m very high risk but will be venturing out once this shielding period ends. I’ve been stuck at home for nearly a year now!

Looking at your details, I suspect you no longer fit the algorithm since your weight loss and lowered Hba1c and the decision should be reconsidered.