Ok Tattlers, case closed, I think I've solved it. My theory is thus - I reckon Mark told Helen whilst he was with her things like "I like redheads" "I like girls who are confident with their bodies" (I can just imagine Helen having a bad body image day and going "Marrrrk I'm fat" and pouting, the way she used to have her strops with Maff. Mark eventually leaves her and finds someone who's actually his type. Good for him.)
So, Helen becomes the girl Mark wanted after he left her. (She also always carries a bit of each ex or friend with her - the gym after Maff, "hiking" after Mark etc, and makes it her identity.)
Having looked through Mark's girlfriend's Insta, she makes the most incredible, decadent looking brownies, looks like it's something she's been doing for a while because they look amazing. What did Helen start doing with Phil? Making really decadent brownies.
These things on their own are insignificant. Who didn't do a little baking in lockdown? 90s flames are on trend now. More than one person can have copper hair. Body positivity is on-trend. Maroon lace underwear is available in the shops. But all together? Creepy AF.