These are just my thoughts.... I have no idea If I am right or Wrong....
...I have never seen a reference to enter a hold at a clearance limit without an onwards clearance. (Happy to read the Jeppesen world wide text for clarification if you can provide a reference, or maybe I have just forgotten some obscure reference)
I would be ensuring I had been given onward clearance instructions. (I have never not been provided with these, and would assume if having been given a STAR you would proceed in accordance with the STAR)
I would not randomly enter a hold unless specifically cleared or I was in non- controlled airspace and made appropriate calls, or if instructions are written on the STAR to enter a hold without clearance.
Most places provide a clearance such as - Cleared the Tommo 1 Alpha Arrival, Rwy XX, Maintain FL380, Expect to Hold at XX. ... Then if required come back with holding instructions.
Do you have a london (presume Heathrow) STAR chart (Jepps) you could provide a reference to, and where you would expect a Jet to enter a hold without clearance.
(I just had a look at that chart Tommo 1 A, and it is a little unusual with the clearance limit and holding pattern at the end....)
I guess if you are having troubles with compliance, why not remove the confusion and make the Clearance crystal clear - that way the pilot knows exactly what is expected, and you don't get any nasty surprises.