This might be a common misconception, but yes, I was right (for once), in the UK you do have to stop at a STOP sign, slowing and giving way isn’t the same thing!
So whether a car, bicycle or other vehicle you must stop at the STOP sign.
For your own safety it is worth noting that I’ve witnessed a lot of drivers who do not stop at a STOP sign or give way at a GIVE WAY sign, so whenever you see a car approaching the junction always be prepared for the worst. In my eyes preparing normally involves cycling further out in the road and hovering my fingers over the front brake (or rear brake in winter).
Know your traffic signs
PDF of the full contents of the ‘Know your traffic signs’ booklet (section: Regulatory signs)
Original post that prompted this one