I got the letter too (I live in Northern Ireland). It appeared to have been sent by my local medical centre, and stated that the doctors there had identified me as a person who was in a high risk category for hospital admission if I catch the virus, and advised me to stay in the house and not go out even to collect prescriptions, which they said would be delivered by my named pharmacy. That was the only concession the letter mentioned. When I rang the pharmacy to let them know that I had received the letter, they said they already knew because the medical centre had passed to them the list of people who had received them. I don't know if the system is the same in England, Scotland or Wales.