Your mandatory period of self-isolation and testing requirements are based on the travel information which you give.
If, before you leave your port of arrival, you are aware that the travel information you have already provided on the Travel Tracker is not true, accurate or up-to-date, you must provide that information on the Travel Tracker found here: https://covid19.gov.gg/guidance/travel/traveltracker
In any other case, this online form must be used to advise the Guernsey Border Agency where you are aware that the travel information you have already provided on the Travel Tracker is not true, accurate or up-to-date.
Following submission of this online form, you will be contacted by a representative of the Guernsey Border Agency to confirm the change in your travel information. Where you have submitted this online form, you should self-isolate (if not already doing so) until you have been informed by that representative whether your self-isolation and testing requirements have changed. If you do not self-isolate and it is found that, because of the change in your travel information, you are required to do so, you may be prosecuted and, if convicted, be sentenced to pay a fine of up to £10,000.