I use the Primark period knickers on lighter days too.
M&S do perfectly good period knickers. They have 3 grades I think - light/med/heavy flow. I have the med and heavy ones for different times of my period. I also use a moon cup for when I don't want big pants on, it's fine. I've used it on and off for about 10 years (not the same one!!). When emptying it you do rather need access to running water so not great in cubicles.
@Terri Thankyou. I was thinking about a menstrual cup but I don’t think that’s something I’d really get on with as I’m not comfortable with tampons so don’t think I’d be too comfortable with putting a menstrual cup up there!