In case the airline will not store my luggage for a 15 hour layover, is there a place in the LHR airport or in Admirals Club to store it?
If you're asking whether they have left luggage facilities, they do, as the Heathrow website mentions:
There are left luggage/baggage storage facilities in each terminal at Heathrow airport. Baggage and luggage items can be stored for a few hours or up to 90 days.
Find prices, locations, and other details on that webpage. (It was the first Google hit by the way.)
If your luggage is checked through to TLV, then you of course do not need to worry about it.
To expand Jonik's answer, here are the current prices for luggage storage at Heathrow:
And here are the opening hours at each terminal: