DP, which airport says you have to reduce traffic to 220kts to achieve a CDA?
I don't recall seeing such an edict for LHR. Given that the night time KPI for LHR with reference to descent starts at 6,000ft, most a/c are likely to be 220kts or less by then anyway without any intervention from me. Keep traffic high as they approach 18 to 20nm from touchdown and all you get is requests on the r/t for descent...
We need to accept that we are not flying the aircraft, the pilot is. We can supply all the information the pilot requires to manage the approach and it's then up to the flight deck to do said managing. If I want I can slow an a/c up to 220kts, 180kts or even less spuriously thinking it will help a CDA be achieved but if they want to they can still dive down and fly level for miles.