It's not just the UK. ICAO PANS-ATM says:
4.5.7.5.1 The flight crew shall read back to the air traffic controller safety-related parts of ATC clearances and instructions which are transmitted by voice. The following items shall always be read back:
a) ATC route clearances;
b) clearances and instructions to enter, land on, take off on, hold short of, cross taxi and backtrack on any runway; and
c) runway-in-use, altimeter settings, SSR codes, level instructions, heading and speed instructions and, whether issued by the controller or contained in automatic terminal information service (ATIS) broadcasts, transition levels.
Note that it only talks about SSR codes not SSR operating instructions - like idents.