Lights, number plates and accessories

Questions:

• A lot of cars have their fog lights on when it's not foggy, is it legal?
• Do I need extension mirrors on my caravan?
• Have you any tips for driving in the winter conditions/fog?
• How dark can I tint the windows on my vehicle?
• I have seen a car with blue neon lights under the vehicle, is it legal?
• I have seen cars with small blue lights on the windscreen wipers, is this legal?
• I have seen some cars with lights on their tyres, is it legal?
• I have seen some spray that can cover my registration number, is it legal?
• I want to have a personalised numberplate with fancy writing, what are the requirements?
• Is it illegal to drive a car with a broken speedometer?
• What are the legal requirements in relation to number plates?