It leaves me wondering if we are not going to mandate the carriage of transponders for all traffic we should mandate its carriage for traffic flying above a certain speed or above a certain altitude (regardless of the class of airspace).
Already is is it not? Mode C compulsory > 10,000' I think:
Conspicuity Code
When operating at and above FL 100 pilots shall select code 7000 and Mode C except:
(a) when receiving a service from an ATS Unit or Air Defence Unit which requires a different setting;
(b) when circumstances require the use of one of the Special Purpose Codes or one of the other specific conspicuity codes assigned
in accordance with the UK SSR Code Assignment Plan as detailed in the table at ENR 1-6-2-5 to ENR 1-6-2-10.
I suspect you mean though wanting a much lower altitude e.g. confining the nasty LAA types to <2000' where they can have even more chance of bumping into each other
NoD