Heathrow Director, read backs are not a universal requirement...not in the US apparently. Im told Taxi instructions dont have to be read back except RWY Hold Short. Clearance Delivery doesnt have to be read back except squawk code. Im told there are no official pilot read-back requirements. I was surprised at this but then again Im told that in NY, you are given a clearance and you dont even have time to read-back, the frequency too busy, you just comply with it. Is this true you americans?