Highway Code Rule 129 "Double white lines where the line nearer to you is solid. This means you MUST NOT cross or straddle it unless it is safe and you need to enter adjoining premises or a side road. You may cross the line if necessary, provided the road is clear, to pass a stationary vehicle, or overtake a pedal cycle, horse or road maintenance vehicle, if they are travelling at 10mph (16km/h) or less." Considering I'm going 21mph, none of those reasons are true. So it's an illegal overtake.