Without any life support systems the tank is no more than a giant bucket. It can only be called an aquarium when the aeration, filtration and temperature control systems are installed. A cooler may also needed if you wish to keep a wide range of British species. This is not the only thing to think about. Fish and invertebrates have to be fed the correct and sufficient amounts of food and attention should be paid to the size they will eventually attain.
Compatibility needs to be considered.
Fish will not only eat each other, but some of the more aggressive animals may be quicker on the uptake and deprive shyer fish of their share. Finally, the sea water in the aquarium needs to be replenished at monthly intervals. Sea Water Tap water mixed with table salt will not do. Sea water contains a large number of additional mineral salts of which many are essential to support life. In addition to sodium chloride these include magnesium, sulphur, potassium, calcium, potassium, bicarbonate and a lot of minor and trace elements.