Look in the UK Air Pilot at the SID charts for all the London TMA airports that have SID's associated (EGLL/KK/GW/SS/LC). Under general information at the top is always a point saying "Max IAS 250KT below FL100 unless otherwise authorised." There is no mention on any of them about passing 4000'. All the STAR charts have Speed Limit Points too where a/c must be <250kts unless otherwise authorised.