The few references on-line that offer any information at all on the keeping of British marine life all agree that lighting is rather unimportant and our dull waters are simply not used to light - a single tube is usually deemed sufficient.
Its my own view that most of the creatures you are likely to keep will be found in rockpools. They will be used to very high levels of light and its probable that even tropical quality halides will not be able to match the levels that may be required for the successful keeping of many algaes found near the shore.
Therefore I use as many lights as I can fit on the lid and boost them with reflectors to maximise the amount of light. Ideally I'd use metal halides but I'm hoping that I can keep algaes successfully under flourescents - time will tell.
This much light under a tight lid generates a lot of heat, I've not been able to find anything anywhere near as good as a chiller for keeping the temperature at the correct level.
I run the lights on a 12 hour cycle with a simple mechanical plug timer, make sure its rated to carry enough current for the number of lights to be used.