Any person who suffers loss or damage, due to the negligence of another person, has the right to sue for compensation. The purpose of insurance cover is to meet that claim (preferably without going through the courts). So the other driver has the right to sue your brother for the damage to his car, for personal injuries and for his loss of earnings. In practice (assuming that your brother is uninsured), the taxi driver will probably make a claim to the Motor Insurers' Bureau. They'll provide him with a payout but then seek compensation, through the courts, from your brother. He'll almost certainly be sued for tens of thousands of pounds but it might be for hundreds of thousands. (If the accident had left someone needing lifetime medical care, the amount could run into millions).
If your brother was driving without insurance, he faces a fine of up to �5000, plus 6 to 8 points on his licence. (That's in addition to the penalties for 'driving without due care and attention' or 'dangerous driving', as appropriate). If your brother had your mother's permission to drive the car, she also faces a fine of up to �5000 and 6 to 8 points on her licence. (The law regards allowing someone to drive without insurance just as seriously as actually doing the driving). If your brother did not have his mother's permission to drive the car, he faces further prosecution for 'taking without consent'.
Chris