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http://www.planningportal.gov.uk/permission/responsibilities/buildingregulations/failure
"First, if a person carrying out building work contravenes the Building Regulations, the local authority may prosecute them in the Magistrates' Court where an unlimited fine may be imposed (sections 35 and 35A of the Building Act 1984). Prosecution is possible up to two years after the completion of the offending work. This action will usually be taken against the person carrying out the work (builder, installer or main contractor)."
with windows, yes. Mostly to show they have the right U value or have safety glass if ground floor or in a door etc.