As far as I can tell, the limit is because of a hand search of every bag at the boarding gate on inbound intl flights. The limit would likely be to make the amount of searching manageable.

I just left LHR on the 24th and avoided this. We'll see if my domestic segment tonight has any changes. It's only about a 60 minute flight wheels up to wheels down, so perhaps I'll have to sit in my seat, not reading or doing anything useful, for the entire flight.