Dutch
Many UK companies whose staff are involved in cash handling ask staff to declare what personal money they have prior to the commencement of work. Some, including ASDA (Wallmart), insist that checkout/till operators do not have any cash on their person at all during working hours and their have been instances of staff being dismissed for disregarding the rule.
I'm not sure there's any law to support it, but if it's company policy then it's part of you contract of employment and in most cases it's common sense purely because it makes life a lot easier if there is any discrepency.
Back to tipping cabin crew.
If I get above average levels of service I will not hesitate to say thank you to the individual and ensure that at least the Cabin Crew Manager is aware, I will also forward the same information to the airline concerned.
Equally I've no objection to change from goods purchased on board going to the airlines nominated charity.
There are some service sectors where tipping is acceptable, but the airline industry is not one of them.
Despite what you may think the average cabin crew salary is significantly more than most waiters, hotel and kitchen staff, all of whom work significantly harder and longer hours than most of you.
Your primary job is ensuring the safety and comfort of the passengers, that's what you are paid for, cabin service is a secondary function and, just as I do my job to the best of my ability for the salary I've negotiated and nothing else, I expect the same from you when I fly as a passenger and I don't see any reason to tip you for it.