Highway Code Rule 129

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, 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

Do you have a particular location in mind, DB?

There is now a plethora of law with the introduction in March of the Traffic Signs and General Directions 2016 which I find hard to read unlike the previous well set out 2002 Directions. Progress??????????????

I would gather that this applies:

Viewed in the direction of travel, if the line closest to you is continuous, you must not cross or straddle it (except to turn into OR OUT OF A SIDE ROAD OR PROPERTY, avoid a stationary vehicle blocking the lane, or overtake a pedal cycle, horse or road works vehicle moving at not more than 10 mph)

Which makes sense other wise one could travel miles until a suitable turning.

dvd

I you look at it purposively, the solid line is intended to prohibit overtakes, or rather those that cross the line. The line also prohibits parking/waiting etc. Reletivley recently exemptions were added for slow moving traffic, including cyclists.

Not intnded to prohbit, or otherwise interfere with, joining or leaving the road.