A car horn should only be sounded and headlights should only be flashed as a warning. Unfortunately, in the UK, car horns are most frequently sounded as a rebuke and headlights are often used to 'flash someone out' or, in the case of HGV drivers, to let the driver of an overtaking truck know it has cleared your vehicle. That's the problem. In the situation you describe, the most sensible thing for your husband to do would have been to lift off his accelerator slightly and let the other driver join, despite the fact that the other driver has no automatic right to force his way onto the motorway.