I see now - confusion on what "Mode S" means!
To LTC, MACC, and soon Prestwick Centre controllers, Mode S Enhanced Surveillance DAP (Downlink Aircraft Parameters) display is a recent addition to the display systems, including items such as magnetic heading, IAS, vertical rate and probably most useful of all - Selected Altitude / Flight Level. This is often referred to as "Mode S" by ATC as it's all the extra information that is provided that wasn't before. The fact that the position, Mode A identity and Flight Level information has been derived / provided by Mode S rather than Mode A/C for quite some time is transparent.
Mode S transponder fit on an aircraft is different and has been an ongoing theme since the 90's. I don't know when the TCAS II mandate came in (before my time?) but when I was involved in the Mode S groundstation pre-operation trials (2001-2002) and deployment of the first operational Mode S groundstation for NATS under the RSS programme (2003 onwards), there was over 85% of all aircraft in cover already Mode S transponder equipped before CAA and other European Mandates for Mode S transponder equippage (started 2003). These were almost all due to the TCAS II requirements. A snapshot today puts Mode S transponder carriage figure at 95% ish - depending on how much GA is in the air
You have to have at least a Mode S Elementary transponder now to enter the London TMA, apart from a few exceptions.
As a complete aside, come 2012, all SSR radars have to be Mode S and all transponders operating in airspace requiring transponder carriage have to be Mode S elementary (including Gliders
!!!!)
For TCAS II to work, you need a Mode S transponder with an appropriate TCAS interface. You do not need to be Enhanced Mode S capable
RS