The cracked windscreen can cause a few issues with the police. In fact, it can attract their attention so that they issue you with a Yellow Sticker. This means that you have to take your vehicle to a Vehicle Examination Centre within 14 days of the expiry date of the sticker. You can also visit an approved inspection station.
If the examiner finds that your vehicle is not safe to be driven, then they can order that it is taken off the road immediately. This is more likely to occur if you have a cracked windscreen that is dangerous and/or if you have further issues with the vehicle. The inspector will check your vehicle over for other defects at the time of your inspection.
If your car is declared as being unfit for the roads, then you best bet is to get your windscreen replaced.