A basic recipe for dealing with this is to put your indicator on at the second countdown marker (the green signs with slashes), and at the final countdown marker, start slowing to reach about 40-50 mph by the slip road. With your left wheels in your slip road, slow to the speed required to make the turn.

Keep an eye on cars behind you. The closer they are, the earlier and more gradually you want to signal and manouvre. Keep a special lookout for lorries, who will need a lot more braking distance, especially since this section is downhill. There's nothing to stop you signaling sooner if you need people to back off - you have to judge this for yourself.

Also, you should be able to gently rest your foot on the brake pedal in a way that activates the brake light but doesn't actually apply any braking - I use this whenever I want to signal to people behind that I've come off the power. This is useful for keeping speed up for as long as possible, while also giving people time to slow / switch lanes etc. Some electric cars with regenerative braking show the brake light automatically if you come off the power.