From 1st April this year, if you fly into ANY airspace where carriage of an operational transponder is required, it has to be Mode S at least to the European Mode S Elementary surveillance requirements. It has already been a requirement for London TMA for some time, but there has been some exemptions which I understand end on 31st March.
While you can operate a Mode A/C only transponder outside of that airspace, it would be preferable change for a Mode S one at some point. There are only a handful of Mode A/C only radars left in the UK now (Mil aside), most are now Mode S (albeit backward compatible with Mode A/C)
RS
