If your vehicle isn’t roadworthy/safe to drive, you could face three penalty points, a fine of up to £2,500 or, in an extreme case, a driving ban. It may also invalidate your insurance if you have an accident and the reason for the accident is due to the vehicle not being roadworthy.
For the full details, you can read the goverment's guidance on how to check your vehicle is safe or review the Highway Code to see what the rules say.
In short, you need to ensure your windscreen, windows, mirrors, reflectors and number plates are clean and you have working lights and brakes before you start any journey.
In addition, you must have fluid in your windscreen washer bottle and also that your tyres are to be correctly inflated, with enough tread depth and without cuts or other defects.
You should also carry out regular checks with your vehicle’s handbook explaining what to look at and when. This includes your oil, radiator level, brake fluid and battery.