Penworth,
I believe the 230kt restriction is because the ROSUN and MIRSI holds are fairly adjacent to each other and it is to stop aircraft arriving at either holding fix 'overshooting' and infringing the other holding pattern. DAYNE is further away and therefore does not have the 230kt restriction.
The POL-BURNI-ROSUN route from the east is also for the same reason.
I believe that you should only proceed beyond any of these positions when instructed to do so by Manchester Radar or Manchester Control, unless carrying out RTF Failure Procedures.
I think the UKAIP still states that aircraft arriving at a holding fix should report their position to ATC.