BA insists on charging you full whack as you are classified as a business-type passenger when not staying a sat night.
I get around this by buying a return ticket from London and a return ticket starting at my destination and using the tickets as follows:
LHR-TLV-LHR - ticket 1 (usual cheap ticket with Sat stay)
TLV-LHR-TLV - ticket 2 (usual cheap ticket with sat stay)
Go out LHR-TLV on ticket 1 (e.g Thursday)
Return 1-2 days later on TLV-LHR on ticket 2 (Friday)
Back to TLV on ticket 2 on next trip and then
TLV-LHR on ticket 1.
Not illegal.
I save £££'s ... does mean you have to go back to TLV again though.