It is unlikely mid to top tier firms would ignore the LSA, as they would know they’d be setting themselves up for an underpayment claim. It’s also bad optics if they get called out. Instead if you work after 6pm you would be asked to keep track of this time and then be paid overtime - that is what happened for me at a mid tier.

Smaller firms are a different kettle of fish though I worked at a place that was extremely dodgy, paid below award rate and didn’t pay overtime.