Yes on 31 Dec 2011 all Mode a/c radars should have been replaced by Mode S.
extract from Cap 761 here:
The environmental benefits of using Mode S interrogators for both civil and military ATS applications are fully recognised by the NISC. Consequently, under Schedule 5 of the Air Navigation Order (ANO) 2005, the UK CAA has mandated the carriage of Mode S transponders for the majority of aircraft operating in designated airspace1. Furthermore, it is developing policy to increase equipage levels and extend Mode S airspace2. The environmental benefits will only be realised fully with widespread Mode S equipage of ground interrogators and airborne transponders. Therefore, the NISC will no longer issue approvals for ground-based (including maritime) or airborne3 IFF/SSR Mode A/C interrogators after 31 December 2011.