Been doing a little more research for you and I think I have found what you are looking for. Anchor lights and signals may apply in certain areas - where, I do not know, but the below refers to Plymouth area so assume similar rules/by-laws apply in areas of heavy shipping movement. The advice on our small boats was taken from the internat so it is useful information to be aware of but will not apply in some areas.https://www.hmso.gov.uk/si/si1999/99202902.htm
2. For the purposes of the Collision Regulations and Rule 13 of this Schedule all the navigable waters of the Dockyard Port north of 50°20'.00 North, that is to say the line passing through Plymouth Breakwater in an east-west direction, shall be deemed to be a 'narrow channel'.
Anchor and other lights and signals (exemptions) etc
3. - (1) Unmanned vessels and vessels with only shipkeepers on board, where secured to moorings out of the fairway, may be exempted by the Queen's Harbour Master from the necessity of carrying anchor lights.
(2) Vessels carrying cargoes of hazardous, dangerous or noxious substances as defined in Regulation 8 of the 1987 Regulations when anchored, moored, or secured alongside within the Dockyard Port shall, in addition to the lights and shapes prescribed in Rule 30 of the Collision Regulations, display the following lights or signals:
(a) between sunset and sunrise, a red all round light visible for 2 nautical miles situated above any other light being displayed by the vessel;
(b) between sunrise and sunset, a red swallow-tailed flag (International Flag Bravo) at the masthead which shall also be displayed when the vessel is underway.
(3) Submarines secured to buoys within the Dockyard Port shall, without prejudice to any requirement prescribed by Rule 30 of the Collision Regulations, display between sunset and sunrise an all round amber light, flashing at a frequency of between 90 and 105 flashes per minute.