The CAA policy on Mode S transponder fit is best summarised as follows:
When operating in airpsace or under any air traffic service where transponder equippage is required, then Mode S SHALL be the means of compliance.
These are my words rather than a direct quote, but are clearer than the AIC and I assure you that this is precisley the emphasis that the CAA intend with their current mandate.
What this does not preclude is the fitting or continued use of Mode A/C purely for purposes of conspicuity (both to ATC and TCAS). Whether you can continue to buy non-Mode S kit is another matter.