This is from the Highway Code



108: Double white lines where the line nearest 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 to pass a stationary vehicle, or overtake a pedal cycle, horse or road maintenance vehicle, if they are travelling at 10mph or less.
Laws RTA sect 36 & TSRGD regs 10 & 26