Most Supermarkets (Inc Sainsburys where I shop) do this
Lets take an example.
Lasagne at �2.99 is one offer,buy one get one free.
If you buy the same Lasgne at �1.99 reduced because of sell by date,the following will happen.
Buying the normal Lasagne X 2 @ �2.99,will give you a saving (buy one get one free) of �2.99.
If the Lasange is reduced to �1.99(sell by date expiry)
you get 2 X �1.99 = �3.98.
The till will automatically deduct the(BOGOF) saving from the reduced price,which gives you:~
�3.98(reduced price) less buy one get one free saving of �2.99,which means you would pay 99p for the 2 Lasagnes.
This applies across the board to all food which has both a BOGOF and a reduced price,to ensure that you do not lose the benefit of the savings.