The limit button is not on the US-spec M5's. The UK-spec ones have a different set of buttons on the left of the steering wheel. One of them is labelled "LIM". It activates the speed limiter set to the current speed. The cruise control +/- buttons can then be used to change this arbitrary ceiling. The car won't accelerate beyond this speed unless you stomp on the lol pedal, or the system is deactivated. If you do mash the lol pedal, the limit remains set, so once you dip below that speed again it will enforce again.
The idea is that you can set the limit to the speed limit of the road you're on, and then drive however you wish, safe in the knowledge that you won't accidentally be speeding.
When I moved to the US, I wondered why the US ones don't have them.