Always frustrating when bogus capacity concerns are cited to deny lounge entry. The OW lounge entry rules don't say anything about discrimination by airline so this isn't really legitimate and I'd complain to BA if you feel suffiiently aggrieved. I'd understand it slightly more on a Saturday which is ski season changeover day, but on a Monday...?

Anyway at least you aren't missing out on too much. The BA lounge is OK but nothing special.