6a) 500 generally, 1000 above built up areas, plus lateral limits etc. Pick up an air law book - that's where all this is covered, or look online for rule 5.
6b) What's the circuit alt - that might be a good clue
Otherwise I'm not 100% sure but I think there'd be a minimum ceiling for VMC - I'd not takeoff under 500ft ceiling's VFR
7) What you call them depends on who you're talking to. I don't know who you'd call for the london TMA, but it's class A, so no VFR anyway!
9) Because you don't really want A/c on the ground producing transponder returns that confuse the radar picture.