Boris was not very clear when he talked about the 12 week thing.
So to be clear, the list at the top is of people who they strongly advise to limit social contact. Mild asthma (ie not needing a steroid inhaler) does not put you on that list. We should all be limiting social contact, but people on the list really should stick to it.
If you have a SERIOUS health condition, you will be contacted by the NHS from Monday 23rd to tell you about extra measures you should take. This is the '12 week quarantine'. If you don't fall into any of the categories here, you don't have to stay indoors for 12 weeks. Pregnant women aren't on this list, so although you should take care, you don't need to be housebound for 3 months.
This list is of people who would be seriously, seriously unwell (sadly, I'm on it) if we were to catch the virus. These are who the respirators will be needed for. So do us all a favour and stick to the rules eh? DH has to go to work, he has no choice currently. So people like us are counting on healthy people to just stay the hell away and limit our risk.