It's an offence to drive a car with tyres with insufficient tread, even if you're genuinely going to a garage to replace them.
If, say, you were to buy a secondhand car with illegal tyres, your only options would be to call a tyre company to come and fit new tyres at the point of purchase or pay for a recovery vehicle to convey your car to a garage. Since many tyre firms offer a roadside service at the same price as they charge at their depot, there's no excuse for driving a vehicle with illegal tyres.
Chris