Its not just the UK law that was changed to allow boarding of ships in International waters. The UN law of the sea was also amended (See Articles 109 and 110

The attitude of the OFCOM and its predecessors towards Radio Caroline has varied througout the years. The official attitude was always that they were doing everything in their power to close the station. But in practice there were occasions when it seemed a certain degree of tolerance (or indifference) was being displayed. Particularly when they were only broadcasting on one low powered frequency and there were no other stations about. At other times the attitude was very different. Although it was even hinted during the events of 18/19 August 1989 that it was the 819 and particularly the 6215 transmissions (even though the latter were not actually on air at the time of the boarding) that had provoked the authorities and that they werent particularly bothered about 558 (despite the issues with RTE and BBC Essex).

Using 6215 KHz had to be the most stupid thing that R Caroline ever done