Signal a u-turn to road users I am living in the UK and in this situation (sorry for my bad drawing):

The bike (the dot in front) wants to do a u-turn and needs to signal to the car (the dot behind). If the bike does not signal, the car will not slow down and my hit the bike.
How can I signal a u-turn? I was thinking about putting my hand behind me but that may not be obvious for road users.
First, don't attempt a u-turn at a junction where you may have vehicles approaching from multiple directions and vehicle drivers will be expecting you to make a left or right turn, not a u-turn.

U turn away at a point where you can see both directions clearly - away from blind turns and rises. Wait until vehicle have passed you to turn, If you need to get vehicles past, stop and wave them by.

You can also more safely u-turn by turning right or left onto a minor side road (or driveway, carpark etc), turning around there, then making the opposite turn back onto the major road.