I am no expert but in absence of any more informed replies, try this:
http://www.caa.co.uk/default.aspx?catid ... geid=12136
In
http://www.ead.eurocontrol.int/eadbasic ... -04-04.pdf
see Section 5.3
ie 5.3.1
You must have Elementary (ie not Enhanced) Mode S for flight in these UK airspaces :
(c) All aircraft within United Kingdom controlled airspace of Classification A, B and C.
(d) All aircraft operating under IFR within United Kingdom controlled airspace of Classification D and E.
(e) All aircraft within United Kingdom airspace at and above FL 100.
(f) All aircraft within United Kingdom airspace notified as a ‘Transponder
Mandatory Zone’. Note: Applies to Airspace Classes D, E, F and G as appropriate .
(g) All aircraft (including EU-OPS aeroplanes) registered in the United Kingdom, flying for the purpose of Public Transport
Note however Exceptions listed in 5.3.4
And Exemptions listed in 5.3.5
Phew!! No wonder a PPL now requires a pass in nine examinations!
So it seems it is not illegal to fit Mode C per se but doing so may restrict your flying to some degree.
Regards
Steve