Driving a car that needs an MOT and doesn't have one: a fine of up to £1,000. No points for this offence on its own, but it's rarely on its own — see below.
If the car has a defect graded dangerous — bald tyre, brake failure, a wheel about to come off — driving it is a separate and more serious offence: up to £2,500, three penalty points, and a possible driving ban. This applies whether or not the car has a current MOT; a dangerous car with a certificate is still a dangerous car.
Most policies require the car to be roadworthy and, in practice, to have a valid MOT. Have an accident without one and the insurer can reduce or refuse a claim — and if they pay out to the other party, they can pursue you for it. Of the three consequences, this is the one that can cost tens of thousands.
You can't renew road tax without a valid MOT. If the MOT lapses and the tax then falls due, you end up untaxed as well, which is a separate fine and a possible clamp.
You may drive a car without a valid MOT directly to a pre-booked MOT test or a pre-booked repair appointment, provided it has no dangerous defects. Book first — you'd need to show the appointment existed if you're stopped. Same-day slots are here.
You can test up to a month early without losing any days, and both gov.uk and we will remind you. There's really no reason to be caught out — check your expiry date now.