Planning on doing my first motorway solo trip on Friday afternoon to Brighton.

Little confused by the speed limit for this. On the A23 London North road as you can see here - maps.app.goo.gl/c1Jz5LpsMomozpLh8

there's a 50mph sign. But then further down as you're joining the M23 there's no further sign. So does this mean the M23 is 50mph too? Guessing not as the motorway means 70mph obviously but why is there no sign saying so as you slip into the M23 (as far as I can see)?