Speed limits apply to motorways as well - Bill Payer
The problem with 50 limits in roadworks is that most HGVs will continue at their limited 56 so you're in their way and they will often squeeze past.
As it happens my car (Mercedes) displays km/h in the dash computer so I can set the speed limiter (or cruise if the traffic is light) for 90km/h and I'm not troubled by HGVs then.