Whether or not the U-Turn was performed illegally (i.e. in a place where they are specifically forbidden) you may still be charged with careless driving if you carried out a manoeuvre which caused a collision.
The likely penalty for the sort of �low-level� careless driving which you describe is a fine of about one week�s net income (plus about �45 costs and a �15 Victim surcharge) and three or possibly four penalty points.
What you describe does not constitute Dangerous Driving as suggested by TWR, even if a death had resulted. (It is the manner of driving which determines the charge, not the outcome). Dangerous Driving is a far more serious matter which results in a mandatory minimum disqualification of twelve months, followed by an extended re-test and possibly a custodial sentence. (Custody is not available for Careless Driving).