There's a couple of very common faults with the Ford Ka. The idle control valve (ICV) and the throttle position sensor (TPS).
Both of these can be found attached to the inlet manifold of the Ka and usually get clogged up with rubbish off the road. I would recommend you get a cheap haynes manual off amazon and give them both a good clean out - especially the ICV (very common). They both cause the erratic rev's when depressing the clutch.
Edit: it's not the act of depressing the clutch that actually causes the issue - it's lifting off the accelerator - just most people press the clutch when lifting off.
Edit2: When you press the clutch you also cause more friction on the engines spline as this is what causes the revs to drop off, the ICV tried to compensate and doesn't accurately do so as it's clogged. Thought I need to explain better