Unfortunately it is not breaking any laws as it clearly states 'we only fill to 300 litres' and the tank is pictured only 3/5ths full - this would keep it within its payload, all the law is worried about. Also it is fixed with a strap, so if stopped by a VOSA inspector they would say you have taken steps to secure the temporary load. However whether it is safe or not is another matter as we all know this is not a temporary load, but a permanent fixture. Currently we are in the middle of a crash test program and part of the initial testing was to demonstrate to ourselves the way standard tanks react in a 30mph crash. Based on this experience such tanks fitted in this way are not really safe at all, most people think that a bulkhead will protect them - it will from small moving items, but not from 300kg of water.
Of course it is breaking a law in another way and anyone who purchases it will be doing so, as the van uses Ionics' logo and clearly states that it uses Ionics' Pure Water System. This is breaking Intellectual Property laws and it would fall foul of the description of goods act.