Braking one-handed to signal down steep traffic-heavy slope
As an confident road rider I'd do two major things.


I'd turn my head, look backward and make eye contact with the driver(s) just behind me to show intention, and maybe signal at this point.  Or I might not signal at all.  Then
take the lane.  You will be moving fast enough to slot-in with the cars because of the downhill.  I'd avoid being sidelined on the side of the road.  I'd also do this well-before the intersection; hard to say how far but even 50-100 metres out.


If I'm needing to brake hard, I'll use both hands, and leave signalling/indicating out of it completely.  You've already shown your intent by taking the lane and moving closer to the center-line.



Since your experience is lesser, feel free to not do any of that.  You have to be comfortable with your intent and actions.  So instead you could:


Get off and walk this bit.  Brake early, loose speed from the downhill, and simply walk across the roads before the intersection.
If permitted, you could ride on the footpath, though the UK doesn't allow this unless it's a specific shared path.
Find another route to work.  There's a fair assumption that the best route for a car is also the best route for a bike, when frequently there are alternatives only available to bikes.


Also, if other cyclists traverse this intersection then watch and observe their methods.  If it makes sense then emulate them, and if not, then don't.