No. Tiny cracks in the tread or sidewall is not illegal to drive with, but it does indicate that your tire is near the end of its life. Cracks can start to appear after about 5 years when exposed to extreme temperatures like standing in the sun all day or icy temperatures.
When these cracks start to get deeper your chances of getting a blowout increase and it is not safe to drive anymore and the tire should be replaced. Cracks in the tire might cause your yearly MOT (UK roadworthiness test) to fail, without the MOT certificate it is illegal to drive your car in the UK.
