How do they know it's an infra-red torch? They won't be able to see it, as you feel infra-red wavelengths as heat, and the companies making these things are advertising torches of a few hundred milliwatts as high power, so you won't feel it

Do you mean an infra-red night vision system, which someone might use to look into dark houses? They often will have an attached IR source. If so, thick curtains are your answer. Any standard insulation measure, to keep your house warm, will keep such spying out.

I know the police carry them as standard kit in a patrol car, but I'm guessing from the list of people UK based sites are trying to sell such sights to, ((casual or professional wildlife observation, conservation, home or professional security, land management, gamekeeping, marine, search and rescue or law enforcement)) snip.
that they are legal.