“ . . . in the UK. A transponder mandatory zone means a mode S
transponder mandatory zone. “

Wrong, at least for the new Stansted TMZ’s:

“ . . . access is permitted to any aircraft which is operating a fully
functioning pressure altitude reporting transponder in accordance with
the CAA’s TMZ Policy Statement or to those aircraft which the aircraft
commander has obtained permission from the air traffic control unit at
Stansted Airport, Farnborough Radar, or Essex Radar, as may be
appropriate, to enter the restricted airspace.”

So it needs Mode C, not necessarily Mode S, thought the latter also
incorporates mode C.

However, for new installations, I believe that Surfer is right – new
installations have to mode S.

If not so, no doubt somebody else will be along soon, To clarify.

Chris N.