There are legal limits specifying how dark tinted windows can be on a vehicle:
Be careful whether the tint you are considering blocks 70% of light or allows through 70% of light. For front windows the regulations are about the amount of light that is allowed through. Also be aware that if you have a factory tint that blocks 10%, you will only be able to block an additional 20%.
Window tinting is not checked on the annual MoT test - the cost of the equipment required at every MoT centre would be excessive. Window and windscreen tints are the responsibility of VOSA. If you are stopped by the police and found to have excessively tinted windows, you will be required to remove the tint. Driving with windows that do no comply with UK window tinting law may invalidate your insurance. Limo style all round tints are illegal in the UK.
The window tinting limits required are primarily for safety reasons. In dull conditions and at night, the driver's ability to see is severely hampered by excessive tints. Not all countries have regulations as strict as the UK. In these countries a number of road deaths have been directly attributed to the driver being unable to see properly through a heavily tinted front windscreen. In addition it is helpful for other road users to be able to see the driver of the vehicle, particularly at junctions.
Motorcyclists are also subject to laws restricting tinted helmet visors.
None of the four cars on the left would be street legal for driving on UK roads. Professional car window tinting companies will be able to advise you on legal tints