This is for London TC.
We can see on a selected mode S pallet on the radar for a selected aircraft, A/C call sign, IAS, ground speed, rate of climb or descent, selected level on FMS, actual level, altimeter setting (I think its from the Captains side only?), heading (actual, not selected) and mode S identifier.
On the radar we always see the selected level of all aircraft.
We also have quick look buttons to see all aircraft IAS, G/S and heading. Using this information, we will be given warnings if the wrong altimeter setting is selected.
We will also be given an indication if the aircraft is outside CAS, but this is not from mode S.
There is also mode S in London AC, they also can see the selected level of aircraft, but I am not sure what else they can see.
Mode S is also used on the ground at Heathrow
No knowledge of other units.