Sorry to be a spoilsport, but I take it you Gents (and Ladies, if appropriate) are aware of the following from the UK AIP GEN 1.5.3. :-
1.3 Carriage of SSR Transponders
1.3.1 The requirements for the carriage of Secondary Surveillance Radar (SSR) equipment are hereby notified for the purpose of
Article 20(2) and Schedule 5 to the Air Navigation Order 2005. With the exceptions listed at paragraph 1.3.3, aircraft shall carry SSR
transponder equipment with a minimum capability as prescribed in the following table:
UK AIP (7 Jun 07) GEN 1-5-3
Civil Aviation Authority AMDT AIRAC 6/07
GEN 1.5 — AIRCRAFT INSTRUMENTS, EQUIPMENT AND FLIGHT DOCUMENTS
GEN 1.5.3 — EQUIPMENT TO BE CARRIED (cont)
(a) All aircraft operating outside of notified Mode S Enhanced Surveillance
airspace:
(1) United Kingdom Airspace at and above FL 100.
(2) United Kingdom Controlled Airspace below FL 100 when
operating under IFR.
(3) The Scottish TMA between 6000 ft ALT and FL 100
(4) When flying for the purpose of public transport.
(b) All aircraft operating inside notified Mode S Enhanced Surveillance
airspace:
(1) When operating in accordance with a Special Flight Notification
and a Rule 21 Flight in Class A Airspace exemption authorized
by the Civil Aviation Authority.
(2) When operating with a valid exemption from the Mode S
carriage requirements at sub-paragraphs 1.3.1 (c) and 1.3.1 (d)
below issued by the Civil Aviation Authority.
Mode A with Mode C altitude reporting.