in week 3. So between day 14-21 and not sure when she started feeling ill, so presumably didn't feel great for longer than that. Which takes us to somewhere between 27 minus 14 = 13th and 27 minus 21 = 6th December.
This tweet says she's had the headaches since around three weeks ago, confirming it's at the earlier end of the date range.
L's tweets (as referred to in this article) confirm she's had symptoms for around three weeks.
She had symptoms from the 10th. A cough. I'll repeat that; a
cough. A cough.
On Saturday, December 12th, the article was published in the Express.
And JM went to buy three copies.
JM was therefore out and about at the very least (ignoring the fact that three weeks' history of headaches could also be her having it first and L was out, working and cycling whilst her partner was unwell) whilst L had one of the unequivocal symptoms that necessitate the entire household to self isolate and anybody with symptoms to arrange for Covid testing. A cough.
ETA: When you test positive, Public Health takes the 48 hour period prior to symptoms as the likely point at which you start infecting other people (it does for schools, at any rate). So anybody in a household where somebody has a cough would know, especially if their job was literally 'to ask questions about Covid-19' that the SI instructions would also apply to people they'd been around for the previous couple of days before developing a cough. I'm sure they would also know that sore throat and headaches are also commonly reported as symptoms, even if they aren't listed by English Government bodies as such. Other countries do include them, which somebody working in the news realm would be well aware of. It seems reasonable that somebody knowing those pesky little facts would know them before getting a positive test result.
So why was JM scampering off to buy the papers, then?