Firstly, road legal means compliant with the Europe wide and/or UK regulations and laws.
The only road legal Quad Bikes (quadricycles as they are formally known as) in the UK are ones which have past very stringent tests with authorities such as the TUV in Germany or the VCA in the UK. This means the vehicle is homologated – so just like a motorbike or car, it has to pass safety tests, strict emissions, and diagnostic displays and meet other specifications in order to be allowed on the road to be registered.
The current top level of these is Euro 4 homologation, which means the ATV or Quad Bike will pass the Euro 4 emission test and will certainly be fuel injected as it would not pass with the old carburettor system so that is the first question you should ask your dealer. Is this quad fuel injected? If not, it cannot have passed Euro 4 and be homologated. This is normally the quad you should buy if you wish to have something for road use.
The second tier of homologation is T1 (T1a and T1b) and T3 (T3a and T3b usually), which is a tractor homologation normally used for side by side vehicles instead of ATVs and quads although there are some quads that have this homologation and are still road legal but if registered as an agricultural machine, you cannot carry passengers and you can only travel for short journeys by road (not more than 1.5 kilometres) between land that’s occupied by the same person. So be careful – there are rules that will restrict the use of your quad depending on how it is registered – so always check. Here’s the information regarding agricultural use vehicles: www.gov.uk/vehicle-exempt-from-vehicle-tax.