Although the UK has voted to leave the EU, this decision does not take immediate effect. More information on the process is available here.
If you are cruising the waters of another country under the United Nations Convention on the Law of the Sea, that country is quite within its rights to ask you to comply with their rules and regulations, should they wish you to do so. However courteous recognition accorded by one nation to the laws and institutions of another means this is seldom the case.
The Coastal State may however require visiting vessels to comply with some of its requirements, the most common of which is requiring the skipper to prove his or her competence for that role. Carriage of local publications or regulations is also commonly stipulated. It is important that you are aware of your obligations prior to leaving UK territorial waters.