Only allowed if you can prove that they died before 1947, illegal for anything after that - and stuff can be carbon-dated to prove age.
Trouble is, the biggest market for ivory is in the Far East, and they are not as fussy out there. If they can ship it out to China, it will be sold on there.
See on this link (dating ivory bit is at Box 5.14) Wildlife crime: a guide to the use of forensic and specialist techniques
jings - you should have been up here in Edinburgh on April 15.
To think that London, or anywhere else for that matter, does not belong to any one demographic