As I understand it the requirement for Mode S in CAS in UK has been deferred to March 2012 provided Mode C is fitted and your a/c details have been 'logged' with the CAA/NATS .There's a thread elsewhere giving site address.
The principal exception to this is London TMA where Mode S continues to be required.
Not that London will let a SEP through anyway, usually preferring a route round the edge.
Just my take.
PS There's a short article on the subject in AOPA's General Aviation magazine August 2009 issue which arrived today:- page 5 .
Cusco
Last edited by Cusco; 27th Jul 2009 at 10:47.