Highway Code Rule 129 Double white lines where the line nearest you is solid

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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 10 mph (16 km/h) or less.

‍Laws RTA 1988 sect 36 & TSRGD regs 10 & 26

This rule includes any part of your vehicle. Including Panniers if you are on a motorcycle.