On the STAR plates UK-AIP for Heathrow, max speed upto FL140 is 220kts, above this 'Standard ICAO' speeds apply. I know this doesn't exactly answer the question, but shows there is a difference at the levels you state. Further to this, the stacks in the TMA (with exception of LAM) are only separated upto FL150, which is maxmium normal used level in each. How this is calculated definately depends on what the leg lengths are stated as, I believe its 1 min unless otherwise authorised and cannot see any authority on the STAR charts, therefore what your chap said is correct in part, but the reason being you aren't procedurally separated from the other stacks!!!!
However I must point out I'm not a TMA controller and am using a VERY dusty blue book, but someone will no doubt correct me quicker if I'm wrong
